Exodus 21:22 describes a situation in which two men fight and injure a pregnant woman, causing her unborn child to leave her womb. The  Masoretic text  uses the Hebrew term "veyats'u yeladeha" (וְיָצְאוּ יְלָדֶיהָ) [ 54 ]  to refer to the child coming out; [ 55 ]  different English versions translate this term either as a "premature birth" or as a "miscarriage". [ 56 ]  The Spanish translation published by the Sociedad Biblica Catolica Internacional (SOBICAIN) uses the term "aborto", clearly indicating the demise of the fetus. [ 57 ]  If no additional harm follows, then the perpetrator must pay a fine. Only if there is additional harm must the perpetrator be punished with equal harm (i.e. eye for an eye). [ 58 ]  Commentators such as  Bruce Waltke  have presented this verse as evidence that God does not value a fetus as a human being, and/or evidence that a fetus has no soul. [ 59 ] [ 60 ] [ 61 ] [ 62 ] [ 63 ]   C. Everett Koop  disagreed with this interpretation. [ 64 ]


Another Old Testament passage that has been used to argue for divine approval of abortion is Numbers 5:11-31, which describes the test of an unfaithful wife. [ 65 ]  If a man is suspicious of his wife's fidelity, he would take her to the high priest. The priest would make a substance for the woman to drink made from water and "dust from the tabernacle floor". If she had been unfaithful "her abdomen will swell and her womb will miscarry, and she will become a curse." If she was innocent the drink had no effect. [ 66 ]


The early Christian work called the  Didache  (before 100 AD) says: "do not murder a child by abortion or kill a new-born infant." [ 67 ]   Tertullian , a 2nd- and 3rd-century  Christian   theologian  argued that abortion should be performed only in cases in which abnormal  positioning  of the fetus in the womb would endanger the life of the pregnant woman.  Saint Augustine , in  Enchiridion , makes passing mention of surgical procedures being performed to remove fetuses which have  died in utero . [ 68 ]


Saint Augustine believed that abortion of a  fetus animatus , a fetus with human limbs and shape, was murder. However, his beliefs on earlier-stage abortion were similar to Aristotle's, [ 69 ]  though he could neither deny nor affirm whether such partially formed fetuses would be resurrected as full people at the time of the Second Coming. [ 70 ]


The  Leges Henrici Primi , written c. 1115, prescribes compensation for a woman or her relatives if another person causes her to miscarry, and prescribes penance (3 years if the abortion occurs before quickening, 7 years after quickening) if the pregnant woman aborts her pregnancy; the latter punishment applied only to women whose abortion resulted from a desire to conceal illicit sex. [ 72 ] : 130–131   "Quickening", a term often used interchangeably with "ensoulment" or "animation", was associated with the first movement of the fetus in utero. This movement is generally felt by women sometime in the third to fifth month of pregnancy.  Midwives  who performed abortions were accused of committing  witchcraft  in  Malleus Maleficarum  ( The Hammer of Witches ), published in 1487 as a  witch-hunting  manual in Germany. [ 73 ]


In 1591,  Pope Gregory XIV  published new regulations in the apostolic constitution  Sedes Apostolica [ a ]  (published on 31 May 1591), limiting the punishments to abortion of a "formed" fetus: [ 74 ] [ 75 ]  "When abortion was neither 'an issue of homicide or of an animate fetus,' Gregory thought it 'more useful' to return to the less-harsh penalties [for early abortion] of the holy  canons  and  profane laws : those who abort an  inanimatus  [soulless] will not be guilty of true homicide because they have not killed a human being in actuality; clerics involved in abortions will have committed mortal sin but will not incur  irregularity ". [ 76 ]  After 1591, Gregory's  Sedes apostolica  "remained in effect for almost three centuries, being revised only in 1869 by Pius IX". [ 77 ] : 148


With his 1869 bull  Apostolicae Sedis moderationi ,  Pope Pius IX  rescinded Gregory XIV's not-yet-animated fetus exception with regard to the spiritual penalty of excommunication.  [ 78 ]  From then on this penalty was incurred automatically through abortion at any stage of pregnancy. [ 79 ]


Currently, the Roman Catholic and Eastern Orthodox churches oppose abortion from conception. [ 80 ] [ 81 ]


From a Jewish perspective from biblical times, abortion is considered from a social perspective more than from a theological perspective. The mother's life is considered as a priority. [ 82 ] [ 83 ]


19th-century medicine saw tremendous advances in the fields of  surgery ,  anaesthesia , and  sanitation . Social attitudes towards abortion shifted in the context of a backlash against the  women's rights  movement. Abortion had previously been widely practiced and legal under  common law  in early pregnancy (until  quickening ), and it was not until the 19th century that the  English-speaking world  passed laws against abortion at all stages of pregnancy. [ 84 ]


There were a number of factors that contributed to this shift in opinion about abortion in the early 19th century. In the United States, where  physicians  were the leading advocates of abortion criminalization laws, some of them argued that advances in medical knowledge showed that  quickening  was neither more nor less crucial in the process of  gestation  than any other step, and thus if one opposes abortion after quickening, one should oppose it before quickening as well. [ 85 ]


Practical reasons also influenced the medical field to impose  anti-abortion laws . For one, abortion providers tended to be untrained and not members of medical societies. In an age where the leading doctors in the nation were attempting to standardize the medical profession, these "irregulars" were considered a nuisance to public health. [ 86 ]  The "irregulars" were also disliked by the more formalized medical profession because they were competition, and often cheap competition. Though the physicians' campaign against abortion began in the early 1800s, little change was made in the United States until after the  Civil War . [ 87 ]


The English law on abortion was first codified in legislation under sections 1 and 2 of  Malicious Shooting or Stabbing Act 1803 . The  Bill  was proposed by the  Lord Chief Justice of England and Wales ,  Edward Law, 1st Baron Ellenborough  to clarify the law relating to abortion and was the first law to explicitly outlaw it. The Act provided that it was an offence for any person to perform or cause an abortion. The punishment for performing or attempting to perform a post  quickening  abortion was the  death penalty  (section 1) and otherwise was transportation for fourteen years (section 2). In the 19th-century United States, there was little regulation of abortion, in the tradition of English common law, pre quickening abortions were considered at most a misdemeanor. These cases proved difficult to prosecute as the testimony of the mother was usually the only means to determine when quickening had occurred. [ 88 ]


The law was amended in  1828  and  1837  – the latter removed the distinction between women who were quick with child (late pregnancy) and those who were not. It also eliminated the death penalty as a possible punishment. The latter half of the 19th century saw abortion become increasingly punished. One writer justified this by claiming that the number of abortions among married women had increased markedly since 1840. [ 89 ]  The  Offences against the Person Act 1861  created a new preparatory offence of procuring poison or instruments with intent to procure abortion. During the 1860s however abortion services were available in New York, New Orleans, Cincinnati, Louisville, Cleveland, Chicago and Indianapolis; with estimates of one abortion for every 4 live births. [ 90 ]


Anti-abortion statutes began to appear in the United States from the 1820s. A  Connecticut  law in 1821 targeted  apothecaries  who sold poisons to women for purposes of abortion, and  New York  made post-quickening abortions a felony and pre-quickening abortions a misdemeanor in 1829. [ 91 ]  Criminalization accelerated from the late 1860s, through the efforts of concerned legislators, doctors, and the  American Medical Association . [ 92 ]  In 1873, the  Comstock Law  prohibited any methods of production or publication of information pertaining to the procurement of  abortion , the  prevention of conception , and the prevention of  venereal disease , even to students of medicine. [ 93 ]  By 1909, the penalty for violating these laws became a $5000 fine and up to five years imprisonment. By 1910, nearly every state had anti-abortion laws; [ 94 ]  these were unevenly enforced at best. [ 95 ] : 80–112


Abortion restrictions are not new, but have increased and spread to places where they were not before. [ 96 ]  Today, there has been an increase in the amount of laws that restrict abortion care in a greater amount of ways that previously seen, and there has been a trend toward abortion restrictive laws and legislature. [ 97 ]  Negative framing around abortion has contributed to today's trend toward anti-abortion legislature. The abortion battle in the United States can be seen as largely a battle of competing ideologies. [ 97 ]  Anti-abortion advocates believe life begins at conception, so legalized abortion is a threat to social, moral, and religious values. [ 97 ]  However, pro-abortion advocates view legalized abortion as a woman's control over her body and the access to safe reproductive care. [ 97 ]


In the United States today, after Dobbs v. Jackson Women's Health Organization, many states have banned abortion and target providers of abortions. In these states where abortion is banned, OBGYNs have been barred from offering abortions or just do not offer the services, except in very limited and certain circumstances, and do not even provide their patients with referrals to other clinicians or online resources involving abortion. [ 98 ]  The reasoning for not offering the services stems from many of the institutions that employ these OBGYNs having policies against performing abortions or terminating a pregnancy, which imposes too many legal regulations involving abortion care. [ 98 ]  OBGYNs have stated that their practices have been impacted and that their relationships with their patients have become worse since the decision on Dobbs was made. [ 98 ]


In contrast, in France social perceptions of abortion started to change. In the first half of the 19th century, abortion was viewed as the last resort for pregnant but unwed women. As writers began to write about abortion in terms of family planning for married women, the practice of abortion was reconceptualized as a logical solution to  unwanted pregnancies  resulting from ineffectual contraceptives. [ 99 ]  The formulation of abortion as a form of family planning for married women was made "thinkable" because both medical and non-medical practitioners agreed on the relative safety of the procedure. [ 99 ]


In New York, surgical abortion in 1800s carried a death rate of 30% regardless of hospital setting, and the  American Medical Association  launched an anti-abortion campaign that resulted in abortion becoming the exclusive domain of doctors. [ 90 ]  A paper published in 1870 on the abortion services to be found in  Syracuse, New York , concluded that the method most often practiced there during this time was to  flush  inside of the uterus with injected water. The article's author, Ely Van de Warkle, claimed this procedure was affordable even to a  maid , as a man in town offered it for $10 on an  installment plan . [ 100 ]  Other prices which 19th-century abortion providers are reported to have charged were much more steep. In Britain, it could cost from 10 to 50  guineas , or 5% of the  yearly income  of a  lower middle class  household. [ 6 ]


From 1870 there was a steady decline in fertility in England, linked by some commentators not to a rise in the use of artificial contraception but to more traditional methods such as withdrawal and  abstinence . This was linked to changes in the perception of the relative costs of  childrearing . Of course, women did find themselves with unwanted pregnancies.  Abortifacients  were discreetly advertised and there was a considerable body of folklore about methods of inducing miscarriages. Amongst working-class women violent  purgatives  were popular,  pennyroyal ,  aloes  and  turpentine  were all used. Other methods to induce miscarriage were very hot baths and  gin , extreme exertion, a controlled fall down a flight of stairs, or veterinary medicines. So-called 'backstreet' abortionists were fairly common, although their bloody efforts could be fatal. Estimates of the number of illegal abortions performed in England varied widely: by one estimate, 100,000 women made efforts to procure a miscarriage in 1914, usually by drugs. [ citation needed ]


A rash of unexplained miscarriages in  Sheffield , England were attributed to  lead poisoning  caused by the metal  pipes  which fed the city's water supply. Soon, women began using  diachylon , a substance with a high concentration of lead, as an abortifacient. In 1898, a woman confessed to having used diachylon to induce a miscarriage. [ 6 ]  The use of diachylon became prevalent in the  English Midlands  up until  World War I . Criminal investigation of an abortionist in  Calgary, Alberta  in 1894 revealed through  chemical analysis  that the concoction he had supplied to a man seeking an abortifacient contained  Spanish fly . [ 101 ]


Dr. Evelyn Fisher wrote of how women living in a  mining  town in Wales during the 1920s used candles intended for  Roman Catholic  ceremonies to dilate the  cervix  in an effort to  self-induce  abortion. [ 6 ]  Similarly, the use of candles and other objects, such as glass rods, penholders,  curling irons , spoons, sticks, knives, and  catheters  was reported during the 19th century in the United States. [ 102 ]  Women of  Jewish descent  in  Lower East Side, Manhattan  are said to have carried the ancient Indian practice of sitting over a pot of steam into the early 20th century. [ 11 ]  Some commentators maintained that abortion remained a dangerous procedure into the early 20th century, more dangerous than childbirth until about 1930. [ 103 ]  But others have said that in the 19th century early abortions under the hygienic conditions in which midwives usually worked were relatively safe. [ 104 ] : 25  [ 105 ] : 4  [ 106 ] : 59
 [ 107 ] : 223  [ 108 ] [ 109 ] [ 110 ]  In addition, some authors have written that, despite improved medical procedures, the period from the 1930s until legalization also saw more zealous enforcement of anti-abortion laws, and concomitantly an increasing control of abortion providers by organized crime. [ 95 ] [ 111 ] [ 112 ] [ 113 ] [ 114 ]


Despite bans enacted on both sides of the Atlantic Ocean, access to abortion continued, as the disguised advertisement of abortion services, abortion-inducing devices, and abortifacient medicines in the  Victorian era  would seem to suggest. [ 115 ]  Apparent print ads of this nature were found in the United States, [ 116 ]  the United Kingdom, [ 6 ]  and Canada. [ 117 ]  A  British Medical Journal  writer who replied to  newspaper  ads peddling relief to women who were "temporarily indisposed" in 1868 found that over half of them were in fact promoting abortion. [ 6 ]


A few examples of surreptitiously marketed abortifacients include "Farrer's Catholic Pills", "Hardy's Woman's Friend", "Dr. Peter's French Renovating Pills", " Lydia Pinkham 's Vegetable Compound", [ 118 ]  and "Madame Drunette's Lunar Pills". [ 6 ]   Patent medicines  which claimed to treat "female complaints" often contained such ingredients as  pennyroyal ,  tansy , and  savin . Abortifacient products were sold under the promise of "restor[ing] female regularity" and "removing from the system every impurity". [ 118 ]  In the vernacular of such advertising, "irregularity", "obstruction", "menstrual suppression", and "delayed period" were understood to be  euphemistic  references to the state of pregnancy. As such, some abortifacients were marketed as  menstrual regulatives . [ 102 ]


Beecham's Pills  were marketed primarily as a  laxative  from 1842. They were invented by  Thomas Beecham  from  St Helens, Lancashire , England. The pills were a combination of  aloe ,  ginger , and  soap , with some other more minor ingredients. The popularity of the pills produced a wide range of testimonials that were used in advertising. The poet  William Topaz McGonagall  wrote a poem advertising the pills, giving his recommendation in verse. [ 119 ]  Beecham's expenditure on advertising went from £22,000 to £95,000 in the 1880s. [ 120 ]  An 1897 advertisement in the  Christian Herald  edition for  Queen Victoria 's  Diamond Jubilee  said: "Worth a guinea a box. Beecham's Pills for all bilious and nervous disorders such as Sick Headache, Constipation, Weak Stomach, Impaired Digestion, Disordered Liver and Female Ailments. The sale is now 6 million boxes per annum." The text was printed alongside a picture of a young woman at a beach and was captioned "What are the wild waves saying? Try Beecham's Pills." [ 121 ]


"Old Dr. Gordon's Pearls of Health", produced by a  drug company  in  Montreal , "cure[d] all suppressions and irregularities" if "used monthly". [ 122 ]  However, a few ads explicitly warned against the use of their product by women who were expecting, or listed  miscarriage  as its inevitable side effect. The copy for "Dr. Peter's French Renovating Pills" advised, "... pregnant females should not use them, as they invariably produce a miscarriage ...", and both "Dr. Monroe's French Periodical Pills" and "Dr. Melveau's Portuguese Female Pills" were "sure to produce a miscarriage". [ 6 ]  F.E. Karn, a man from Toronto, in 1901 cautioned women who thought themselves pregnant not to use the  pills  he advertised as "Friar's French Female Regulator" because they would "speedily restore menstrual secretions." [ 122 ]  Historian  Ann Hibner Koblitz  comments that "Nineteenth-century customers would have understood this 'warning' exactly as the sellers intended: as an advertisement for an abortifacient preparation." [ 123 ] : 44


In the mid 1930s abortifacients drugs were marketed in the United States to women by various companies under various names such as Molex Pills and Cote Pills. Since birth control devices and abortifacients were illegal to market and sell at the time, they were offered to women who were "delayed". The recommended dosage constituted seven grains of ergotin a day. These pills generally contained ingredients such as ergotin, aloes, Black Hellebore. The efficacy and safety of these pills are unknown. In 1940 the FTC [ 124 ]  deemed them unsafe and ineffective and demanded that these companies cease and desist selling these products.


A well-known example of a Victorian-era abortionist was  Madame Restell , or Ann Lohman, who over a forty-year period illicitly provided both surgical abortion and abortifacient pills in the northern United States. She began her business in New York during the 1830s, and, by the 1840s, had expanded to include  franchises  in  Boston  and  Philadelphia . It is estimated that by 1870 her annual expenditure on advertising alone was $60,000. [ 6 ]  Because of her reputation,  Restellism  became a synonym for abortion. [ 125 ]


One ad for Restell's medical services, printed in the  New York Sun , promised that she could offer the "strictest confidence on complaints incidental to the female frame" and that her "experience and knowledge in the treatment of cases of female irregularity, [was] such as to require but a few days to effect a perfect cure". [ 126 ]  Another, addressed to  married  women, asked the question, "Is it desirable, then, for parents to increase their families, regardless of consequences to themselves, or the well-being of their offspring, when a simple, easy, healthy, and certain remedy is within our control?" [ 127 ]  Advertisements for the "Female Monthly Regulating Pills" she also sold vowed to resolve "all cases of suppression, irregularity, or stoppage of the menses, however obdurate". [ 126 ]  Madame Restell was an object of criticism in both the respectable and  penny presses . She was first arrested in 1841, but, it was her final arrest by  Anthony Comstock  which led to her  suicide  on the day of her trial April 1, 1878. [ 127 ]


Such advertising aroused criticisms of  quackery  and  immorality . The safety of many nostrums was suspect and the  efficacy  of others non-existent. [ 102 ]   Horace Greeley , in a  New York Herald  editorial written in 1871, denounced abortion and its promotion as the "infamous and unfortunately common crime– so common that it affords a lucrative support to a regular guild of professional murderers, so safe that its perpetrators advertise their calling in the newspapers". [ 116 ]  Although the paper in which Greeley wrote accepted such advertisements, others, such as the  New York Tribune , refused to print them. [ 116 ]   Elizabeth Blackwell , the first woman to obtain a  Doctor of Medicine  in the United States, also lamented how such ads led to the contemporary synonymity of "female physician" with "abortionist". [ 116 ]


Abortifacient advertising was highly effective in the  United States , though apparently less so across the  Atlantic . Contemporary estimates of mid-19th century abortion rates in the United States suggest between 20% and 25% of all pregnancies in the United States during that era ended in abortion. [ 128 ]  This era also saw a marked shift in those who were obtaining abortions. Before the start of the 19th century, most abortions were sought by unmarried women who had become pregnant  out of wedlock . But, out of 54 abortion cases published in American medical journals between 1839 and 1880, over half were sought by married women, and of the married women well over 60 percent already had at least one child. [ 129 ]  In the  post-Civil War era , much of the blame was placed on the burgeoning  women's rights movement . [ citation needed ]


Many feminists of the era were opposed to abortion. [ 130 ] [ 131 ]  In  The Revolution , operated by  Elizabeth Cady Stanton  and  Susan B. Anthony , an anonymous contributor signing "A" wrote in 1869 about the subject, arguing that instead of merely attempting to pass a law against abortion, the root cause must also be addressed. Simply passing an anti-abortion law would, the writer stated, "be only mowing off the top of the noxious weed, while the root remains. [...] No matter what the motive, love of ease, or a desire to save from suffering the unborn innocent, the woman is awfully guilty who commits the deed. It will burden her conscience in life, it will burden her soul in death; But oh! thrice guilty is he who drove her to the desperation which impelled her to the crime." [ 131 ] [ 132 ] [ 133 ] [ 134 ]  To many feminists of this era, abortion was regarded as an undesirable necessity forced upon women by thoughtless men. [ 135 ]  Even the "free love" wing of the feminist movement refused to advocate abortion and treated the practice as an example of the hideous extremes to which modern marriage was driving women. [ 136 ]  Marital rape and the seduction of unmarried women were societal ills which feminists believed caused the need to abort, as men did not respect women's right to abstinence. [ 136 ]


Socialist feminists tended to be more sympathetic to the need for abortion options for the poor, and indeed socialist feminist doctors, such as  Marie Equi , Madeleine Pelletier, and William J. Robinson, themselves performed low-cost or free abortions for poor women. [ 137 ] [ 138 ] [ 139 ]


The movement to liberalize abortion laws emerged in the 1920s and '30s as part of rising feminist activism that had already resulted in victories in the area of  birth control . Campaigners including  Marie Stopes  in England and  Margaret Sanger  in the US had succeeded in bringing the issue into the open, and birth control clinics were established which offered family planning advice and contraceptive methods to women in need.


In 1929, the  Infant Life Preservation Act  was passed in Britain, which amended the law ( Offences against the Person Act 1861 ) so that an abortion carried out in good faith, for the sole purpose of preserving the life of the mother, would not be an offence. [ 140 ]


Stella Browne  was a leading birth control campaigner, who increasingly began to venture into the more contentious issue of abortion in the 1930s. Browne's beliefs were heavily influenced by the work of  Havelock Ellis ,  Edward Carpenter  and other  sexologists . [ 141 ]  She came to strongly believe that working women should have the choice to become pregnant and to terminate their pregnancy while they worked in the horrible circumstances surrounding a pregnant woman who was still required to do hard labour during her pregnancy. [ 142 ]  In this case she argued that doctors should give free information about birth control to women who wanted to know about it. This would give women agency over their own circumstances and allow them to decide whether they wanted to be mothers or not. [ 143 ]


In the late 1920s Browne began a speaking tour around England, providing information about her beliefs on the need for accessibility of information about birth control for women, women's health problems, problems related to puberty and sex education and high maternal morbidity rates among other topics. [ 141 ]  These talks urged women to take matters of their sexuality and their health into their own hands. She became increasingly interested in her view of the woman's right to terminate their pregnancies, and in 1929 she brought forward her lecture "The Right to Abortion" in front of the  World Sexual Reform Congress  in London. [ 141 ]  In 1931 Browne began to develop her argument for women's right to decide to have an  abortion . [ 141 ]  She again began touring, giving lectures on abortion and the negative consequences that followed if women were unable to terminate pregnancies of their own choosing such as: suicide, injury, permanent invalidism, madness and blood-poisoning. [ 141 ]


Another prominent feminist to influence abortion law was  Emily Stowe . In the 19th century she was one of the first doctors to be tried for attempting an abortion procedure in Canada. [ 144 ]


Other prominent feminists, including  Frida Laski ,  Dora Russell ,  Joan Malleson  and  Janet Chance  began to champion this cause – the cause broke dramatically into the mainstream in July 1932 when the  British Medical Association  council formed a committee to discuss making changes to the laws on abortion. [ 141 ]  On 17 February 1936,  Janet Chance , Alice Jenkins and  Joan Malleson  established the  Abortion Law Reform Association  as the first  advocacy  organisation for abortion liberalization. The association promoted access to  abortion in the United Kingdom  and campaigned for the elimination of legal obstacles. [ 145 ]  In its first year ALRA recruited 35 members, and by 1939 had almost 400 members. [ 145 ]


The ALRA was very active between 1936 and 1939 sending speakers around the country to talk about Labour and Equal Citizenship and attempted, though most often unsuccessfully, to have letters and articles published in newspapers. They became the most popular when a member of the ALRA's Medico-Legal Committee received the case of a fourteen-year-old girl who had been raped, and received a termination of this pregnancy from Dr.  Joan Malleson , a progenitor of the ALRA. [ 145 ]  This case gained a lot of publicity. However, once the war began, the case was tucked away and the cause again lost its importance to the public.


In 1938,  Joan Malleson  precipitated one of the most influential cases in British abortion law when she referred a pregnant fourteen-year-old  rape  victim to  gynaecologist   Aleck Bourne . He performed an abortion, then illegal, and was put on trial on charges of procuring abortion. Bourne was eventually acquitted in  Rex v Bourne  as his actions were "an example of disinterested conduct in consonance with the highest traditions of the profession". [ 146 ]  This court case set a precedent that doctors could not be prosecuted for performing an abortion in cases where pregnancy would probably cause "mental and physical wreck".


Finally, the  Birkett Committee , established in 1937 by the British government "to inquire into the prevalence of abortion, and the law relating thereto", recommended a change to abortion laws two years later. The intervention of  World War II  meant that all plans were shelved. [ 147 ]


Another prominent figure in the reform of abortion laws was  Dr. Morgentaler . Although born in Poland he made a name for himself in Canada, opening multiple illegal abortion clinics in Toronto, Ontario. [ 148 ]


Prior to 1969, abortion was considered a crime for which the maximum punishment was life imprisonment for the doctor performing the abortion and two years imprisonment for the woman receiving the abortion. Abortion remained illegal until 1988, when the Supreme Court of Canada overruled the criminal punishments for abortion. [ 149 ]  Abortion remains a hotly debated topic. As of 2008 in Canada only 1–2% of abortions were pharmaceutically induced. [ 150 ]  After much controversy, starting in 2017 abortion pills could be used legally in Canada. [ 151 ]


The  Russian Soviet Federative Socialist Republic  was the first government to legalize abortion and make it available on request, often for no cost. [ 152 ] [ 153 ]  The Soviet government hoped to provide access to abortion in a safe environment performed by a trained doctor instead of  babki . [ 154 ]  While this campaign was extremely effective in the urban areas (as much as 75% of abortions in Moscow were performed in hospitals by 1925), it had much less effect on rural regions where there was neither access to doctors, transportation, or both and where women relied on traditional medicine. [ 155 ]  In the countryside in particular, women continued to see  babki , midwives, hairdressers, nurses, and others for the procedure after abortion was legalized in the Soviet Union. [ 156 ]


From 1936 until 1955 the Soviet Union made abortion illegal (except for medically recommended cases) again, stemming largely from  Joseph Stalin 's worries about population growth. Stalin wanted to encourage population growth, as well as place a stronger emphasis on the importance of the family unit to  communism . [ 157 ]


During the  Spanish Civil War , on 25 December 1936, in  Catalonia , free abortion was legalized during the first 12 weeks of pregnancy with a decree signed by  Josep Tarradellas ,  First Minister of the Government of Catalonia , and published on 9 January 1937 ( Diari Oficial de la Generalitat de Catalunya, núm.9 ). [ 158 ] [ 159 ] [ 160 ]


In Britain, the  Abortion Law Reform Association  continued its campaigning after the War, and this, combined with broad social changes brought the issue of abortion back into the political arena in the 1960s. President of the  Royal College of Obstetricians and Gynaecologists   John Peel  chaired the committee advising the  British Government  on what became the  1967 Abortion Act . On the grounds of reducing the amount of disease and death associated with illegal abortion, the Abortion Act allowed for legal abortion on a number of grounds, including to prevent grave permanent injury to the woman's physical or  mental health , to avoid injury to the physical or mental health of the woman or her existing child(ren) if the pregnancy was still under 28 weeks, or if the child was likely to be severely physically or mentally handicapped. The free provision of abortions was provided through the  National Health Service . [ 161 ]


In America an abortion reform movement emerged in the 1960s. In 1963, the  Society for Humane Abortion  was formed, providing women with information on how to obtain and perform abortions. [ 162 ]  In 1964  Gerri Santoro  of Connecticut died trying to obtain an illegal abortion and her photo became the symbol of the abortion rights movement. Some women's rights activist groups developed their own skills to provide abortions to women who could not obtain them elsewhere. As an example, in Chicago, a group known as " Jane " operated a floating abortion clinic throughout much of the 1960s. Women seeking the procedure would call a designated number and be given instructions on how to find "Jane". [ 163 ]


In the late 1960s, a number of organizations were formed to mobilize opinion both against and for the legalization of abortion. The forerunner of the  NARAL Pro-Choice America  was formed in 1969 to oppose restrictions on abortion and expand access to abortion. [ 164 ]  In late 1973 NARAL became the National Abortion Rights Action League. The American Medical Association, the American Bar Association, the American Academy of Pediatrics, the California Medical Association, the California Bar Association, and numerous other groups announced support behind new laws that would protect doctors from criminal prosecution if they performed abortions under rigid hospital controls. In 1967,  Colorado  became the first state to decriminalize a doctor performing an abortion in cases of rape, incest, or in which pregnancy would lead to permanent physical disability of the woman.


A bipartisan majority in the California legislature supported a new law introduced by Democratic state senator  Anthony Beilenson , the "Therapeutic Abortion Act". Catholic clergy were strongly opposed but Catholic lay people were divided and non-Catholics strongly supported the proposal.  Governor Ronald Reagan  consulted with his father-in-law, a prominent surgeon who supported the law. He also consulted with  James Cardinal McIntyre , the Catholic archbishop of Los Angeles. The archbishop strongly opposed any legalization of abortion and he convinced Reagan to announce he would veto the proposed law since the draft allowed abortions in the case of birth defects. The legislature dropped that provision and Reagan signed the law, which decriminalized abortions when done to protect the health of the mother. [ 165 ] [ 166 ] [ 167 ]  The expectation was that abortions would not become more numerous but would become much safer under hospital conditions. In 1968 the first full year under the new law there were 5,018 abortions in California. The numbers grew exponentially and stabilized at about 100,000 annually by the 1970s. 99.2% of California women who applied for an abortion were granted one. One out of every three pregnancies was ended by illegal abortion. The key factor was the sudden emergence of a woman's movement that introduced a very new idea—women had a basic right to control their bodies and could choose to have an abortion or not. Reagan by 1980 found his support among anti-abortion religious groups and said he was too new as governor to make a wise decision. [ 168 ]


In 1970, Hawaii became the first state to legalize abortions on the request of the woman, [ 169 ]  and New York repealed its 1830 law and allowed abortions up to the 24th week of pregnancy. Similar laws were soon passed in Alaska and Washington. A law in Washington, D.C., which allowed abortion to protect the life or health of the woman, was challenged in the Supreme Court in 1971 in  United States v. Vuitch . The court upheld the law, deeming that "health" meant "psychological and physical well-being", essentially allowing abortion in Washington, DC. By the end of 1972, 13 states had a law similar to that of Colorado, while Mississippi allowed abortion in cases of rape or incest only and Alabama and Massachusetts allowed abortions only in cases where the woman's physical health was endangered.


The landmark judicial ruling of the  Supreme Court  in  Roe v. Wade  ruled that a Texas statute forbidding abortion except when necessary to save the life of the mother was unconstitutional. The immediate result was that all state laws to the contrary were null. The Court arrived at its decision by concluding that the issue of abortion and abortion rights falls under the  right to privacy . The Court held that a right to privacy existed and included the right to have an abortion. The court found that a mother had a right to abortion until viability, a point to be determined by the abortion doctor. After viability a woman can obtain an abortion for health reasons, which the Court defined broadly to include psychological well-being.


From the 1970s, and the spread of  second-wave feminism , abortion and  reproductive rights  became unifying issues among various women's rights groups in Canada, the United States, the Netherlands, Britain, Norway, France, Germany, and Italy. [ 170 ]


On June 24, 2022,  Roe v. Wade  was overturned by the Supreme Court in a 6–3 decision. The ruling was part of  Dobbs v. Jackson Women's Health Organization , a decision of the Supreme Court that also overturned  Planned Parenthood v. Casey , another case of the Supreme Court regarding abortion. [ 171 ]


Although prototypes of the modern  curette  are referred to in ancient texts, the instrument which is used today was initially designed in France in 1723, but was not applied specifically to a gynecological purpose until 1842. [ 172 ]   Dilation and curettage  has been practiced since the late 19th century. [ 172 ]


The 20th century saw improvements in abortion technology, increasing its safety, and reducing its  side-effects .  Vacuum  devices, first described by the Scottish obstetrician  James Young Simpson  in the 19th century, allowed for the development of  suction-aspiration abortion . [ 172 ]  The process was improved by the Russian doctor S. G. Bykov in 1927, where the method was used during its period of liberal abortion laws from 1920 to 1936. The technology was also used in China and Japan before being introduced to  Britain  and the United States in the 1960s. [ 172 ]  The invention of the  Karman cannula , a flexible  plastic   cannula  which replaced earlier metal models in the 1970s, reduced the occurrence of perforation and made suction-aspiration methods possible under  local anesthesia . [ 172 ]


In 1971,  Lorraine Rothman  and  Carol Downer , founding members of the feminist self-help movement, invented the Del-Em, a safe, cheap suction device that made it possible for people with minimal training to perform early abortions called  menstrual extraction . [ 172 ]


Swedish researchers began testing potential  abortifacients  in 1965. In 1968, the Swedish physician  Lars Engström  published a paper on a clinical trial, conducted at the women's clinic of  Karolinska Hospital  in Stockholm, of the compound F6103 on pregnant Swedish women with the aim of inducing abortion. It was the first clinical trial of an abortion pill to be conducted in Sweden. [ 173 ]  The paper, originally titled  The Swedish Abortion Pill , was renamed to  The Swedish Postconception Pill , due to the small number of induced abortions that occurred in the trial population. After these efforts were largely unsuccessful with F6103, the same researchers attempted to find an abortion pill with  prostaglandins , capitalizing on the number of well-established prostaglandin scientists working in Sweden at the time; they were eventually awarded the  1982 Nobel Prize in Physiology  for their work. [ 174 ]


Abortions brought about by taking medications, known as  medication abortions , became an alternative to surgical aboritons when  prostaglandin analogs  were introduced in the 1970s. One such analog is  carboprost , which was successfully trialed in the United States in 1979. [ 175 ] [ 176 ] [ 177 ] [ 178 ] [ 179 ]


In 1981, French pharmaceutical company  Roussel-Uclaf  developed the antiprogestogen  mifepristone  (also known as RU-486), which successfully induces abortions. It is typically taken in combination with  misoprostol . [ 180 ] [ 181 ] [ 182 ]


Intact dilation and extraction  was developed by Dr. James McMahon in 1983. It resembles a procedure used in the 19th century to save a woman's life in cases of obstructed labor, in which the fetal skull was first punctured with a perforator, then crushed and extracted with a  forceps -like instrument, known as a  cranioclast . [ 183 ] [ 184 ]


Medication abortions became common when mifepristone was approved for use in China and France in 1988.  Later it was approved in Great Britain in 1991, in Sweden in 1992, in Austria, Belgium, Denmark, Finland, Georgia, Germany, Greece, Iceland, Israel, Lichtenstein, Luxembourg, Netherlands, Russia, Spain, and Switzerland in 1999, in Norway, Taiwan, Tunisia, and the United States in 2000, and in 70 additional countries from 2001 to 2023. [ 185 ]


During the mid-1990s in the United States the medical community showed renewed interest in  manual vacuum aspiration  as a method of early surgical abortion. This resurgence is due to technological advances that permit early pregnancy detection (as soon as a week after conception) and a growing popular demand for safe, effective early abortion options, both surgical and medical. An innovator in the development of early surgical abortion services is Jerry Edwards, a physician, who developed a protocol in which women are offered an abortion using a handheld vacuum syringe as soon as a positive pregnancy test is received. This protocol also allows the early detection of an ectopic pregnancy. [ 172 ]


As of 2022, medication abortions are more common than surgical abortions in most places around the world. [ 181 ] [ 186 ]


At various times abortion has been banned or restricted in countries around the world. Multiple scholars have noticed that in many cases, this has caused women to seek dangerous, illegal abortions underground or inspired trips abroad for "reproductive tourism". [ 187 ] [ 188 ] [ 189 ]  Half of the world's current deaths due to unsafe abortions occur in Asia. [ 187 ]


Other authors have written that illegality has not always meant that abortions were unsafe. In the U.S. during the 19th century, early abortions under the hygienic conditions in which midwives usually worked were relatively safe. [ 104 ] : 25  : 4  [ 106 ] : 59  [ 107 ] : 223  [ 108 ] [ 109 ] [ 190 ]


In the early 1950s, the Chinese government made abortion illegal, with punishments for those who received or performed illegal abortions written into the law. [ 191 ]  These restrictions were seen as the government's way of emphasizing the importance of population growth. [ 191 ]


As the decade went on, the laws were relaxed with the intent of reducing the number of deaths and lifelong injuries women sustained due to illegal abortions, as well as serving as a form of population control when used in conjunction with birth control. [ 191 ]  In the early 1980s, the state implemented a form of family planning which used abortion as a "back-up method"; and in 2005, there has been legislation trying to curb sex-selective abortion. [ 191 ]  As of 2009, although China had the highest number of abortions in the world, Russia had the highest rate in the world. [ 192 ]


India enforced the  Indian Penal Code  from 1860 to 1971, criminalizing abortion and punishing both the practitioners and the women who sought out the procedure. [ 189 ]  As a result, women died in an attempt to obtain illegal abortions from unqualified midwives and "doctors". [ 189 ]  Abortion was made legal under specific circumstances in 1971, but as scholar S. Chandrasekhar notes, lower class women still find themselves at a greater risk of injury or death as a result of a botched abortion. [ 189 ]


In 2023 state reported 500,000 abortions performed in the year against 1.5 million births. [ 193 ] [ 194 ]


Japan is known today worldwide for its acceptance of abortion. [ 187 ] [ 195 ] Since induced abortion was legalized in 1948, The numbers of annual abortions increased to 1.17 million in 1955, making the induced abortion rate per 1000 women aged 15-49 50.2. The number dropped in 2005 to total number 289,000, making the abortion rate per 1000 women 10.2. Social factor and medical advance including contractive are giving effect to the number. [ 196 ] 。


The Eugenics Protection Law of 1948 made elective abortion care legal up to twenty-two weeks' gestation so long as the woman's health was endangered; in 1949, this law was extended to consider the risk the child's birth would place on a woman's economic welfare. [ 187 ] [ 195 ]  Originally, each case would have to be approved by a local eugenics council, but this was removed from the law in 1952, making the decision a private one between a woman and her physician and woman's partner. [ 187 ] [ 195 ] [ 197 ]
。In 1996, the law was partly revised, and the name was changed to Maternal Health Act. The law still requires partner's agreement for abortion, in some cases leaving women who need abortion without. [ 198 ] 。


In 1964, the creation of the conservative right-wing nationalist political lobbying group called  Seicho-no-Ie  brought about a strong opposition to the abortion laws. [ 187 ]  This campaign reached its peak strength in the early 1980s, but ultimately failed in 1983. [ 187 ]


In 1957, Romania legalized abortion, but in 1966, after a decline in the national birthrate,  Nicolae Ceauşescu  approved Decree 770, which criminalized abortion and encouraged childbirth. As a result of this decree, women in want of abortion turned to illegal procedures that caused the deaths of over 9,000 women and left unwanted children abandoned in orphanages. Abortion remained illegal until 1989, when the decree was overturned. [ 199 ]


There was intense public debate throughout the 1980s and 1990s over legal abortion reform. [ 187 ]  These debates portrayed abortion as un-Buddhist and anti-religious; abortion opponents ultimately labeled it as a form of Western corruption that was inherently anti-Thai and threatened the integrity of the nation. [ 187 ]  Despite this, in 2006, abortions became legal in cases of  rape  or  foetal impairment . [ 187 ]  Mental health also became a factor in determining the legality of an abortion procedure. [ 187 ]  The strict regulations involved in qualifying for a legal abortion, however, cause approximately 300,000 women a year to seek illegal avenues according to scholar Andrea Whittaker, with the poorest undergoing the most dangerous of procedures. [ 187 ]


Human reproductive ecology  is a subfield in  evolutionary biology  that is concerned with human reproductive processes and responses to ecological variables. [ 1 ]  It is based in the  natural  and  social sciences , and is based on theory and models deriving from human and animal biology,  evolutionary theory , and  ecology . It is associated with fields such as  evolutionary anthropology  and seeks to explain human reproductive variation and adaptations. [ 2 ]  The theoretical orientation of reproductive ecology applies the theory of natural selection to reproductive behaviors, and has also been referred to as the evolutionary ecology of human reproduction. [ 3 ]


Multiple theoretical foundations from evolutionary biology and evolutionary anthropology are important to human reproductive ecology. Notably, reproductive ecology relies heavily on  life history theory , energetics,  fitness theories ,  kin selection , and theories based on the study of animal evolution.


Life history theory  is a prominent analytical framework used in evolutionary anthropology, biology, and reproductive ecology that seeks to explain growth and development of an organism through various life history stages of the entire lifespan. The life history stages include early growth and development, puberty, sexual development, reproductive career, and post-reproductive stage. Life history theory is based in evolutionary theory and suggests that  natural selection  operates on the allocation of different types of resources (material and metabolic) to meet the competing demands of growth, maintenance, and reproduction at the various life stages. [ 4 ]  Life history theory is applied to reproductive ecology in the theoretical understandings of puberty, sexual growth and maturation, fertility, parenting, and senescence because at every life stage organisms are bound to encounter and cope with unconscious and conscious decisions that hold trade-offs. [ 5 ]  Reproductive ecologists have specifically impacted life history by improving on the energetic models because they are complicated in humans, and involve many causal factors. They draw on classical life history theory,  behavioral ecology , and reproductive ecology to make predictions about reproductive behavior and growth [ 6 ]


Analytical frameworks that explore problems relevant to reproductive ecology, such as age at  menarche , or  lactational amenorrhea , often employ understandings of energetics to their hypotheses and models. [ 7 ]  Energetics in this context refers to energy allocation, under the assumption that natural selection favors optimal allocation and use of energy, but also that trade-offs often pose energetic constraints. Allocations of energy are evolved so they in turn, can be foreseeable but they are also variable depending on ecological constraints.


The assumption that energy measured in calories can be used as a universal measure of nutritional cost is criticized by a number of scientists on the basis of  essential nutrients , nutrients that the body cannot produce regardless of calorie availability and the specific nutrients must be present in the diet. It is argued that since there are different dietary conditions in which different essential nutrients are the most scarce in different regions and the few foods that contain the scarcest nutrients that are needed to avoid  deficit diseases  are therefore the most expensive (the cost may be paid in the form of  other goods and services  in societies without money), and different functions in the body primarily consume different essential nutrients, no universal ranking of the costs of different aspects of reproduction can be made. For example, it is possible for the few micronutrients that men consume more of the more sperm they produce but the consumption of which does not increase in women during pregnancy or lactation to be the scarcest nutrients contained in the most expensive food in some societies, making sperm production effectively more expensive than pregnancy and lactation under local food prices in such societies. It is also argued that the variability of what food is the most valuable due to containing the rarest essential nutrients extend their effects to the economical significance ratio between hunting and gathering in the case of hunter-gatherer societies, and therefore that any attempt to circumvent the  evolutionary psychology  paradox of men not being able to be in two places at the same time to hunt and protect his family by reference to hiring guards by bartering meat would fail to make sex roles universal due to the difference between regions where the rarest essential nutrients were contained in one or more types of meat and regions where the rarest such nutrients were contained in some types of plants. It is cited in this context that humans evolved over relatively large parts of Africa with different food ecologies, making it impossible for humans to have specialized evolutionarily for one specific food cost ratio. This variability of food value ratios within Africa may have prepared humans evolutionarily to be able to leave Africa. [ 8 ] [ 9 ]


The researchers involved in human reproductive ecology use the combined approach of demography and evolutionary biology to explain the reproductive phenomenon. Biodemography is the study of demography related to biology and  evolutionary biology . [ 10 ]  Biodemographers do research on demographic outcomes such as  conception ,  spontaneous abortion , births, marriage, divorce,  menarche ,  menopause , aging, and mortality. Biodemographers use mathematical models, statistical estimates and biomarkers to analyze the demographic data. [ 11 ]  The field of biodemography often explores the scientific questions associated with fertility and mortality across cultures, the determinants of reproductive senescence, mortality and sex differences, low fertility in humans, and longer post-reproductive lifespan in women. [ 12 ]


In human reproductive ecology, the study of  pregnancy  is primarily focused variation in pregnancy and on rates of pregnancy loss.


Pregnancy varies person-to-person and across cultural and socioeconomic lines. Human gestation is between 30 and 40 weeks long. [ 13 ]  The dynamic between the mother and the  fetus  is one of conflict: it is in the best interest of the fetus to gestate as long as possible to continue receiving the nutritional and developmental benefits of being physically attached to the mother. For the mother, however, pregnancy is a highly demanding and risky time. Earlier births avoid complications in the birth of a too-large infant. The length of the pregnancy is a compromise between these two demands, and is influenced by factors such as socioeconomic status, health, and fetal development. Women of lower socioeconomic status have been shown to deliver their babies earlier on average than women of higher socioeconomic status. [ 14 ]  Research has also shown that stress, especially during early pregnancy, can cause shorter gestation length and increase premature births. [ 15 ]


The rate of  embryo loss  changes throughout pregnancy. Pre-implantation in the uterine wall, rate of loss is undetectable as the  hCG  hormone is not secreted until  implantation . [ 16 ]  There is no current way to detect pregnancy or  pregnancy loss  at this stage. Post-implantation, rate of loss is highest in the first trimester of a pregnancy. [ 14 ]  The chance of pregnancy loss lowers the further into gestation a woman is. [ 14 ]


Pregnancies may be unsuccessful for multiple reasons. The maternal immune system, though suppressed during ovulation, views the fertilized egg as a foreign body and will attack it. [ 17 ]  Defective embryos may also be spontaneously aborted, or miscarried, whether due to chromosomal abnormality or developmental defects. Endometrial or placental development issues may also cause a pregnancy to fail. Additionally, the frequency of spontaneous abortion increases with the mother's age. [ 18 ]  Older mothers have a higher rate of genetic abnormalities that can trigger pregnancy loss. [ 19 ]


Because human pregnancy is so costly, and human offspring so dependent on their mothers, early spontaneous abortion is high to ensure that energy of a pregnancy is spent on developing a fetus with a high chance of survival.


Human reproductive ecology considers fecundity and fertility from a demographic perspective. In this view,  fecundity  is the reproductive  potential  of an individual and  fertility  is the actual reproductive  output  of an individual.


Fecundity is determined by the biological limitations of the individual and can be reduced when biological and ecological factors impact an individual's reproductive capabilities. The key components of fecundity are a person's reproductive maturation and the maintenance of their reproductive system. In humans, the timing of female reproductive maturation is particularly variable and is heavily influenced by ecological considerations. In addition, the age at menarche has decreased over time in many global populations. [ 20 ]  This phenomenon is referred to as the secular trend. Age at menarche is one measure of the fecundity of an individual female. Male reproductive maturity is less subject to environmental and ecological factors, and does not follow the secular trend that female puberty does.


In adults, fecundity is determined by the biological processes of reproduction. Female fecundity is heavily influenced by reproduction and energetics. The  ovarian cycle  limits the potential of conception to a brief period of fertility roughly once a month. Successful egg maturation, fertilization, and implantation must be able to occur for a reproductively mature female to be fecund. Changes in energy levels, diet, and hormones can all interfere in this process. During breastfeeding, a period of lactational infertility also reduces female fecundity. The metabolic load hypothesis in human reproductive ecology describes how the energetic expenditure of lactation acts to inhibit ovarian cycling. With the majority of available energy going towards milk production, energy is not expended on reproductive effort.


Male fecundity is primarily determined by the quality of sperm and the availability of fertile female mates. Individual variation in sperm load, pH, lifespan, and morphology creates varying fecundity in males. As males do not gestate, their contribution to fecundity is less well established post-reproduction.


A lack of fecundity in adults can be described as  infecundity  or  infertility . Infertility occurs in about 10–15% of couples, [ 21 ]  with the causes of infertility shared equally between males and females.


Fertility is the measure of an individual's actual reproductive output, rather than just their potential for  reproductive success . Fertility rates vary both inter- and intra-culturally.  Fertility for both males and females is dependent not just on biology but on cultural, religious, economic, and other sociological factors as well.


Natural fertility  is emphasized in the study of human reproductive ecology. Natural fertility is the measure of human fertility in populations without birth control. Research on natural fertility populations seeks to understand the evolutionary context, ecological constraints, and predict outcomes for human fertility.


Fertility is influenced by fecundity, but has additional factors that can increase or decrease an individual's lifetime reproductive success. The inter-birth interval, the amount of time between a woman's births, impacts a woman's total fertility. This amount of time varies cross-culturally, as well as varies with different environmental constraints. Many cultures practice conscious  birth spacing  to adhere to the desired length of time between pregnancies, or desired number of children. Environmental concerns like fetal loss, lack of resource access, and disease may all impact fertility for females or males.


Fertility rates across the globe have steadily declined. [ 22 ]  This trend, known as the  demographic transition , began in the 1700s and continues today. It is strongly correlated with increased  industrialization  in a society. This trend is now seen in almost all cultures, resulting in some societies with below replacement fertility. Below replacement fertility is when the rate of childbirth in a society is less than the amount needed for each woman to have at least one daughter. Since the chance of having a daughter is 50/50, there must be at least two children for every adult woman in the population.


In 1961, French demographer Louis Henry introduced the term "natural fertility". [ 23 ]  Natural fertility is defined as uncontrolled fertility when the couples do not control the number of children and the family size. Controlled fertility populations use controlled methods to stop having children after reaching a certain number of children.
In natural fertility populations, the parity related controls of fertility are not influenced by modern birth controls. Therefore, studying and understanding the age-related changes in fecundity is easier in natural fertility populations in compare to controlled fertility populations. Natural fertility populations deliver an easier platform to study the reproductive behavior which may affect the levels of fertility such as pregnancy loss, time for conception, and length of breastfeeding. [ 24 ]  In Pennsylvania and Ohio states in the United States, the  Amish  settlements have been studied to understand the age of marriage, the age of first birth, birth intervals, the age at last birth, and total fertility rate as they are natural fertility population due to their religious belief. [ 25 ]  The  Dogon  population in Mali, West Africa are a natural fertility population with high fertility rate and they have been studied to understand the role of the age of wife, the age of husband, nutritional status, breastfeeding status, sex of last child, economic status, and polygyny on the waiting time to conception. [ 26 ]  Natural fertility population in rural  Bangladesh  have been studied to predict the role of parity, pregnancy loss, mother's age, economic status, child's sex, and husband's migration on the distribution of postpartum amenorrhea. [ 27 ]


The number of children in any family is associated with the quality of those children. [ 28 ]  There is a trade-off between reproduction and survival of the children which influence total fertility rate in humans globally> [ 29 ]  In  sub-Saharan African  countries, child survival is negatively associated with the number of children in the family due to the child competition for  parental investment . The decrease in birth interval rate can also endanger the life of the child. In the  Hungarian  population, a shorter birth interval is associated with less investment of the mothers which results in small body size and low birth weight of the children at birth. [ 30 ]  In historical Ireland (1700–1919) the number of children in the family was negatively associated with the lifespan and reproductive success of the children. [ 31 ]  In various natural fertility populations, the shorter birth interval length may cause higher deaths of the infants. The hunter-gatherer ! Kung  mothers require to carry a greater amount of food and baby on foraging trips and shorter birth interval length results higher  infant mortality  among them. [ 32 ]  The 4-year birth interval is the optimum for the !kung women to have a maximized reproductive success. The total fertility of the women is also related to the post-reproductive survival of the women [ 33 ]  and in pre-industrial (1766–1895) Swedish population, the number of children was found be to be negatively associated with the longevity of the mothers. [ 34 ]


Puberty  is the transitory stage in human development in which a person goes from a child into a reproductively mature adult, in other words, puberty is the process of sexual maturation in humans. The onset of puberty varies between boys and girls, with boys usually starting around 11–12 years of age and ending by 16–17, [ 35 ] [ 36 ] [ 37 ] [ 38 ] [ 39 ]  and girls starting around 10–11 and ending at 15–17. [ 40 ] [ 20 ] [ 41 ]  Activity in the  hypothalamic–pituitary–gonadal axis (HPG axis)  initiates puberty by secreting  gonadotropin-releasing hormone  (GnRH) from the  hypothalamus  into the  anterior pituitary . The anterior pituitary releases the  gonadotropins   luteunizing hormone (LH)  into the  ovaries , which produce  estrogen , and  follicle-stimulating hormone  (FSH) into the  testes , which produce  testosterone . The central event in puberty for females is  menarche , the first  menstrual  bleeding. For males, it is the first  ejaculation . The onset of menarche is easier to determine due to the evidence of menstrual bleeding, while the first ejaculation for males is usually self reported. In evolutionary context, it is assumed that  human physiology  has been modeled through  natural selection  to maximize reproductive success by allotting energy and resources through  trade-offs . [ 14 ]


This period of reproductive maturation sees the onset of  primary sexual characteristics , the production of  gametes  and hormones by the  gonads , and  secondary sexual characteristics . Secondary sexual characteristics include  adolescent   growth spurt ,  pubic  and axillary hair,  genital  enlargement,  breast development  in girls,  beard  growth in boys, increase in  subcutaneous fat , increase in  muscle  mass, and widening of the  pelvis  in girls. While there is variation among individuals, secondary sexual characteristics tend to develop in a sequence. For girls, breast development is followed by the appearance of pubic hair, followed by menarche, and fat deposition and broadening of the hips occurring as the completion of breast development approaches. For boys, enlargement of the  penis  and testicles occurs, followed by pubic and axillary hair growth, voice change, facial hair growth, and muscle mass increase. [ 42 ]  This period is also a time of cognitive and psychosocial development where social relationships, skills, and experiences outside of the core family are explored. [ 14 ]


While puberty is a consistent progression of events culminating in reproductive maturity, there is wide variation in age of onset of puberty and the magnitude of the changes that can be caused by a variety of different influences. Since the mid 19th century the global age of menarche has significantly decreased. [ 43 ] [ 44 ] [ 45 ]  Dietary composition, disease, psyschosocial circumstances, developmental conditions, genetics and epigenetics, and other environmental factors can all affect the age of the onset of puberty. [ 46 ]  These factors can come together and in terms of evolutionary trade offs, alter the allocation of energy into growth, maintenance, or reproduction, as best needed for survival. Most research focuses on female puberty because it is easier to determine due to menarche. While there is variation in the  onset time and magnitude, the sequence of events stays more or less consistent, variations in the sequence can indication a pathological condition.


Differences in quality and quantity of nutrition account for one of the strongest environmental factors that alter the onset of puberty. [ 47 ]  Evidence has linked childhood obesity in girls with early pubertal timing, referencing an increased amount of body fat as a signal for the brain to initiate puberty and due to an excess of available energetic resources, since developing a  fetus  is very energetically demanding. [ 48 ]


Disease and chronic illness in childhood can lead to a delay in pubertal timing in boys and girls.  Inflammatory diseases ,  parasitic infections , and other illnesses that affect nutritional intake, specially chronic ones, are energetically costly and energy and resources has to be allocated into maintenance and health, sometimes taking energy from growth or reproduction, stunting or delaying them.


Variation in pubertal timing has been directly found to be due through direct genetic association between mothers and daughters in 46% of the population studied. It is believed that an  androgen receptor   gene , but the specific gene has not been found. [ 47 ] [ 49 ] [ 50 ] [ 51 ] [ 52 ]  Chemicals and hormones found in the environment [ 53 ]  and plastics such as  Bisphenol A (BPA) [ 54 ]  have been thought to affect sexual development in humans at the prenatal or postnatal stage. According to the  Centers for Disease Control and Prevention  (CDC), BPA found in plastic bottles and containers leaches into foods and liquids when warmed up, as in the case of plastic baby bottles, and traces of the chemical were found in more than 90% of the U.S. population studied. BPA is of concern because it interferes with the actions of  estrogen  which is needed as a developmental and reproductive regulator.


Most of the studies have reported that menarche may occur a few months earlier in girls in high-stress households, whose fathers are absent during their early childhood, who have a stepfather in the home, who are subjected to prolonged  sexual abuse  in childhood, or who are  adopted  from a developing country at a young age. Conversely, menarche may be slightly later when a girl grows up in a large family with a biological father present. However, when the stress is severely high and potentially life-threatening such as in times of war, the onset of puberty has been delayed.


Mate choice  in human reproductive ecology is the process by which individuals rationally partner with others. Mate choice practices, like many of the topics in human reproductive ecology, vary greatly between individuals and between cultures.


Culture heavily influences mate choice, but there are evolutionary concepts that underpin research into mate choice. Honest signals are characteristics of an individual that are assumed to be true indicators of health and fecundity. Honest signals guide  sexual selection , the process by which certain traits are picked by the potential mate and then proliferate throughout a species. Human cultures vary on what is considered to be a desirable honest signal. Emphasis on wealth, aesthetics, religious affiliation, and lineage, to name a few examples, are all used in different cultures as ways to choose a mate.


Monogamy  is the mating strategy of two individuals partnering exclusively with each other for a period or time or for life. Monogamy in humans is generally accompanied by selective mate-choice and mating, cohabitation, and bi-parental care for children. Humans may practice lifelong monogamy, as well as serial monogamy. Serial monogamy is the mating strategy of having sequential, non-overlapping partners.


Polygamy  is the practice of having multiple partners at the same time. The composition of the relationship will determine which type of polygamy is being practiced.  Polygyny  is the practice of a male partnering with multiple females. It is a fairly common mating strategy in humans, as well as in many other animals. Polygyny often occurs in agricultural societies and is often paired with male wealth or land access. When males are able to disproportionately control resources, they may be able to support more than one female partner.  Polyandry  is the practice of a female partnering with multiple males. It is not as common in humans as polygyny, due in part to the constraints of female reproduction. While a female may only reproduce once at a time, a male may be able to contribute to multiple concurrent pregnancies. Polyandry is often seen in cases when there are more males in a society than females, or when males are considered to be unavailable.


In reproductive ecology, concepts related to parenting, social organization, and development are discussed. The concept of  parental investment  defined by Trivers and Willard [ 55 ]  in the 1970s is used widely in reproductive ecology to analyze and understand provisioning strategies and how they relate to life history trade-offs. Trivers' parental investment [ 56 ]  is defined as investment in offspring to that benefits their survival and ability to reproduce, at the expense of the parent's ability to invest in other offspring. Inherent in these strategies is an underlying trade-off between energy and investment allocation to oneself as a parent and to each offspring.


Paternal investment  is more variable than maternal investment worldwide, and compared to other  primates [ 57 ]  paternal investment is more robust in humans. Mating and  pair-bonding  includes trade-offs such as making a choice between investing in current offspring, or investing in future mating opportunities. Over the course of human evolution, there is evidence of reduced  sexual dimorphism  in humans compared to other primates. This suggests that there was less male-male competition for female mates, which led to more male investment in offspring, rather than mate choices. [ 55 ]  Paternal investment strategies vary facultatively based on alloparental care, the costs and benefits of offspring investment, societal pressures, divisions of labor, cultural expectations and norms, and the individual qualities of males in any given society. [ 58 ]  In the field of reproductive ecology, it has been a recent interest to explore the  endocrinology  of social relationships, including the relation of paternal investment and endocrine function. [ 59 ]  It has been shown that fatherhood in general, reduces testosterone levels and competition for mates increases testosterone. It is also shown that male endocrine function is mediated by interactions with children. [ 60 ]


Maternal investment  is widespread and less variable than paternal investment, but there have been recent evidence supporting multiple mating systems for females as well in the evolutionary literature. [ 61 ]  This could suggest that mating systems may influence how maternal investment is given, and the trade-offs posed both biologically and socially. Maternal investment is almost always necessary for the survival of offspring, because compared to other primates, human infants are highly altricial. [ 62 ]  Offspring are also categorized as taking longer to wean, still dependent after weaning, and a longer juvenile period. [ 63 ]


Parent offspring conflict  is a theory synthesized by  Trivers  in the 1970s alongside parental investment. Parental offspring conflict is also well documented and develops in tandem with the process of reproduction and parenting. Parent-offspring conflict occurs in the relationship between parent and fetus (in the case of striking a balance between allocating placental energy stores to the growing fetus, while maintaining and metabolic balance of the mothers biology), and between parent and offspring. Parent offspring is expected to be highest during the parental investment period. [ 64 ]   Parent-offspring conflict  assumes there will be "disagreements" between parents and offspring about how long parental investment lasts, how resources are allocated, and maintaining the life history trade-offs in the process. [ 64 ]


Parental investment provided by  individuals other than mothers and fathers is considered allocare. Both paternal care and allocare can reduce the energetic costs of parenting for mothers. Allocare is often referred as allomaternal care or  allomothering  if it is provided by anyone other than the mother. Based on  kin selection theory , it is usually assumed that mothers have been ancestrally necessary to ensure offsprings survival and reproduction. It is less known to what extent  paternal investment  or care or other types of allocare are a necessity to offspring survival and reproduction. Typically maternal care is defined at the most basic level of pregnancy and birth and lactation, but includes other things like provisioning, learning (in humans), mirroring (mirroring behavior of mother), and holding, carrying, and touching. It has been shown in various studies that allocare can take many forms such as provisioning, providing food, reducing parental costs for parents, time investments, economic investments, and other types of care such as holding. There have been different results from studies in traditional societies and natural fertility populations, than in industrialized societies. [ 65 ]  Allomaternal care has been hypothesized to have influenced ancestral evolution by being associated with increased brain size. [ 66 ]  Allomaternal care is also a part of a larger hypothesis of humans as  cooperative breeders  whereby allocare discounts the individual costs of parenting, especially when sets of parents have children around the same time as each other, or have other kin or community members to provide care (see  grandmother hypothesis ). Cooperative breeding is a social system that given some advantage over time, and cooperative breeding is much more common in humans and relatively rare in other mammalian species. Traits in our species that favor cooperative breeding evolve over time due to altruism, and within the context of kin selection and reciprocity.


Lactation is one of the costliest forms of parental investment because it is taxing at a metabolic and physiological level, but also in terms of time and emotion as well. There are many trade-offs regarding lactation, and recent work has explored cost benefit models and thresholds for breastfeeding. [ 67 ]  From a biological and evolutionary perspective, breastfeeding infants is biologically superior and contains various bioconstituents that provide nutrition, hydration, immune factors, hormones, and other necessary components to aid infant survival and growth. Lactational strategies vary cross-culturally, but can typically be defined by sibling sets and sex ratios, frequency of nursing, entire lactational duration, and milk composition. [ 16 ]   Milk  is composed of my bioconstituents, but only a few will be outlined here. In the first days of puerperium, the first milk is thick and yellowish, also called  colostrum . [ 16 ]  For weeks after that, mature milk is expressed and it has been shown that fetal-mammary gland signaling occurs even before birth in determining milk type and concentrations based on the fetus sex. Colostrum plays an important role in establishing the infant  gut microbiome , as it contains important  immunoglobins , and is high in protein and low in fat and milk sugar such as lactose. [ 16 ]  While breastmilk is extremely important for infants' health outcomes, it is also known that human mature milk is fairly dilute, which has an effect on infant  suckling  behavior, which in many cases holds implications for the contraceptive properties of lactation. [ 16 ]


Post-partum infecundability, also referred to as lactational infecundability or  lactational amenorrhea , refers to the section of the human birth interval from parturition to the first post-partum ovulation. [ 16 ]  This period varies widely across globe and between societies. The length of post-partum infecundability is heavily influenced by breastfeeding because it holds some contraceptive physiological effects. [ 16 ]  The role of lactational amenorrhea has been shown to be important for infant survival as a mechanism to delay the next pregnancy, and thus infants have a longer period to optimize nutritional and immunological benefits of breast milk. [ 16 ]  Post-partum hormonal levels change so that both estrogen and progesterone are "cleared from the maternal circulation" [ 16 ]  and without breastfeeding, levels of plasma FSH and LH gradually increase and lead to the return of regular menses within 2 months. [ 16 ]  With breastfeeding, the resumption of normal menses occurs many months later, and the overall effect of lactational amenorrhea is influenced by the intensity of infant suckling. [ 16 ]


Ovarian aging is characterized by the gradual decline of the  ovarian follicles  number and decreasing quality of oocytes.  Menopause  is considered as the final stage of ovarian aging. [ 68 ]  Menopause is clinically defined as the absence of menstruation beyond a year. It indicates the cessation of reproductive phase of life in women. The biology of menopause is associated with the depletion of the ovarian follicular pool. At the fourth month of the fetal life, the ovarian follicles reach to the number 6–7 million. [ 69 ]  At birth, the number of ovarian follicles in the ovary decline to 1–2 million. The follicle number decrease to 300,000–400,000 at the age of menarche. In the entire reproductive age, these follicles undergo atresia and at the time of menopause, the ovaries are left with approximately 1000 follicles. [ 70 ]  Below this threshold regular ovarian cycles cannot be maintained. The quality of ovarian follicles declines with age due to the increase meiotic non-disjunction. After age 31 years, fecundity decreases and the probability of aneuploidy rate increases in the early embryo.


The regular menstrual cycle is associated with the hormonal regulation from hypothalamic, pituitary, and ovarian axis. Gonadotropin-releasing hormone (GnRH) secretes from the hypothalamus. Hypothalamic GnRH pulse influences the pulsatile secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. [ 71 ]  During the  menstrual cycle , due to a decreased level of inhibin-A and steroid hormones, the level of FSH increases. [ 72 ]  Due to these hormonal changes, the corpus luteum gets destroyed. The elevated level of FSH helps to recruit a cohort of the FSH-sensitive antral follicles in that cycle. [ 73 ]  During this phase, elevated FSH level stimulates the production of estradiol,  inhibin  A and B. Following that, due to the negative feedback mechanism the level of estradiol and inhibin-B increases and FSH level declines and it helps to select the dominant follicle. During the menopausal transition, FSH level elevates at the early follicular phase and due to the increased FSH level, the number of FSH-sensitive follicles decreases. [ 74 ]  These series of events lead to irregular menstrual cycle and the cycle length starts to become shorter. FSH, Inhibin-B, and  anti-Müllerian hormone  (AMH) are used as the  biomarkers  for the ovarian aging. [ 75 ]


Various genetic and endocrine factors influence the aging of the ovaries and the age at menopause. In some women, the ovaries age faster and the follicle pool diminishes before the age of 40 years. [ 76 ]  This phenomenon is known as the premature ovarian failure (POF) and it is used as the model for the study of the genetics of ovarian aging. The genes such as GDF9 and BMP15 have been identified as the candidate genes for POF. [ 77 ]  POF has a relation with the genome-wide linkages on chromosomal regions 9q21.3 and Xp21.3. [ 78 ]  Several genes related to mitochondrial function such as mt-Atp6, Sod1, Hspa4, and Nfkbia are also associated with the aging of the ovary. [ 79 ]  In addition to that, the deletion at mtDNA 4977-bp in the granulosa cells is associated with fertility in older aged women. [ 80 ]


Sperm competition  is a form of post-copulatory  sexual selection [ 1 ]  whereby male  sperm  simultaneously physically compete to fertilize a single  ovum . [ 2 ]  Sperm competition occurs between sperm from two or more rival males when they make an attempt to fertilize a female within a sufficiently short period of time. [ 3 ]  This results primarily as a consequence of  polyandrous   mating systems , or due to  extra-pair copulations  of females, which increases the chance of  cuckoldry , in which the male mate raises a child that is not genetically related to him. [ 1 ] [ 3 ] [ 4 ]  Sperm competition among males has resulted in numerous physiological and psychological adaptations, including the relative size of testes, the size of the sperm midpiece, prudent sperm allocation, and behaviors relating to sexual coercion, [ 1 ] [ 3 ] [ 4 ] [ 5 ]  however this is not without consequences: the production of large amounts of sperm is costly [ 6 ] [ 7 ]  and therefore, researchers have predicted that males will produce larger amounts of  semen  when there is a perceived or known increase in sperm competition risk. [ 3 ] [ 7 ]


Sperm competition is not exclusive to humans, and has been studied extensively in other primates, [ 5 ] [ 8 ] [ 9 ]  as well as throughout much of the animal kingdom. [ 10 ] [ 11 ] [ 12 ]  The differing rates of sperm competition among other primates indicates that sperm competition is highest in primates with multi-male breeding systems, and lowest in primates with single-male breeding systems. [ 5 ] [ 8 ]  Compared to other animals, and primates in particular, humans show low-to-intermediate levels of sperm competition, suggesting that humans have a history of little selection pressure for sperm competition. [ 5 ]


Physiological evidence, including testis size relative to body weight and the volume of sperm in ejaculations, suggests that humans have experienced a low-to-intermediate level of selection pressure for sperm competition in their evolutionary history. [ 4 ] [ 5 ]  Nevertheless, there is a large body of research that explores the physiological adaptations males do have for sperm competition. [ 3 ]


Evidence suggests that, among the  great apes , relative testis size is associated with the breeding system of each primate species. [ 13 ]  In humans, testis size relative to body weight is intermediate between monogamous primates (such as gorillas) and promiscuous primates (such as chimpanzees), indicating an evolutionary history of moderate selection pressures for sperm competition. [ 4 ]


The volume of sperm in ejaculates scales proportionately with testis size and, consistent with the intermediate weight of human males' testes, ejaculate volume is also intermediate between primates with high and low levels of sperm competition. [ 4 ]  Human males, like other animals, exhibit prudent sperm allocation, a physiological response to the high cost of sperm production as it relates to the actual or perceived risk of sperm competition at each insemination. [ 14 ]  In situations where the risk of sperm competition is higher, males will allocate more energy to producing higher ejaculate volumes. [ 14 ]  Studies have found that the volume of sperm does vary between ejaculates, [ 15 ]  and that sperm produced during copulatory ejaculations are of a higher quality (younger, more motile, etc.) than those sperm produced during masturbatory ejaculates or nocturnal emissions. [ 3 ]  This suggests that, at least within males, there is evidence of allocation of higher quality sperm production for copulatory purposes. Researchers have suggested that males produce more and higher quality sperm after spending time apart from their partners, implying that males are responding to an increased risk of sperm competition, [ 16 ]  although this view has been challenged in recent years. It is also possible that males may be producing larger volumes of sperm in response to actions from their partners, or it may be that males who produce larger volumes of sperm may be more likely to spend more time away from their partners. [ 3 ]


The size of the sperm midpiece is determined in part by the volume of mitochondria in the sperm. [ 17 ]  Sperm midpiece size is tied to sperm competition in that individuals with a larger midpiece will have more mitochondria, and will thus have more highly motile sperm than those with a lower volume of mitochondria. [ 17 ]  Among humans, as with relative testis size and ejaculate volume, the size of the sperm midpiece is small compared to other primates, and is most similar in size to that of primates with low levels of sperm competition, supporting the theory that humans have had an evolutionary history of intermediate levels of sperm competition. [ 5 ]


Several features of the anatomy of the human penis are proposed to serve as adaptations to sperm competition, including the length of the penis and the shape of the penile head. By weight, the relative  penis size  of human males is similar to that of chimpanzees, although the overall length of the human penis is the largest among primates. [ 18 ]  It has been suggested by some authors that penis size is constrained by the size of the female reproductive tract (which, in turn is likely constrained by the availability of space in the female body), and that longer penises may have an advantage in depositing semen closer to the female cervix. [ 19 ]  Other studies have suggested that over our evolutionary history, the penis would have been conspicuous without clothing, and may have evolved its increased size due to female preference for longer penises. [ 20 ]


The shape of the glans and coronal ridge of the penis may function to displace semen from rival males, although displacement of semen is only observed when the penis is inserted a minimum of 75% of its length into the vagina. [ 21 ]  After allegations of female infidelity or separation from their partner, both men and women report that men thrusted the penis more deeply and more quickly into the vagina at the couple's next copulation. [ 21 ]


In addition to physiological adaptations to sperm competition, men also have been shown to have psychological adaptations, including certain copulatory behaviors, behaviors relating to sexual coercion, investment in relationships, sexual arousal, performance of oral sex, and  mate choice .


Human males have several physiological adaptations that have evolved in response to pressures from sperm competition, such as the size and shape of the penis. [ 21 ]  In addition to the anatomy of male sex organs, men have several evolved copulatory behaviors that are proposed to displace rival male semen. For example, males who are at a higher risk of sperm competition (defined as having female partners with high reproductive value, such as being younger and physically attractive) engaged more frequently in semen-displacing behaviors during sexual intercourse than men who were at a lower risk of sperm competition. [ 22 ]  These semen-displacing behaviors include deeper and quicker thrusts, increased duration of intercourse, and an increased number of thrusts. [ 21 ] [ 22 ]


Men who are more invested into a relationship have more to lose if their female partner is engaging in extra-pair copulations. [ 3 ]  This has led to the development of the cuckoldry risk hypothesis, which states that men who are at a higher risk of sperm competition due to female partner infidelity are more likely to sexually coerce their partners through threatening termination of the relationship, making their partners feel obligated to have sex, and other emotional manipulations of their partners, in addition to physically forcing partners to have sex. [ 23 ]  In forensic cases, it has been found that men who rape their partners experienced cuckoldry risk prior to raping their partners. [ 24 ]  Additionally, men who spend more time away from their partners are not only more likely to sexually coerce their partners, but they are also more likely to report that their partner is more attractive (as well as reporting that other men find her more attractive), in addition to reporting a greater interest in engaging in intercourse with her. [ 25 ]  Men who perceive that their female partners spend time with other men also are more likely to report that she is more interested in copulating with him. [ 25 ]


Sperm competition has also been proposed to influence men's sexual fantasies and arousal. Some researchers have found that much pornography contains scenarios with high sperm competition, and it is more common to find pornography depicting one woman with multiple men than it is to find pornography depicting one man with multiple women, [ 26 ]  although this may be confounded by the fact that it is less expensive to hire male pornographic actors than female actors. [ 27 ]  Kilgallon and Simmons documented that men produce a higher percentage of motile sperm in their ejaculates after viewing sexually explicit images of two men and one woman (a sperm competition risk) than after viewing sexually explicit images of three women, likely indicating a response to an active risk of sperm competition. [ 28 ]


It is unknown whether or not men's willingness and desire to perform oral sex on their female partners is an adaptation. [ 3 ]  Oral sex is not unique to humans, [ 29 ] [ 30 ] [ 31 ]  and it is proposed to serve a number of purposes relating to sperm competition risk. Some researchers have proposed that oral sex may serve to assess the reproductive health of a female partner [ 32 ]  and her fertility status, [ 33 ]  to increase her arousal, thereby reducing the likelihood of her having extra-pair copulations, [ 34 ]  to increase the arousal of the male to increase his semen quality, and thereby increase the likelihood of insemination, [ 35 ]  or to detect the presence of semen of other males in the vagina. [ 32 ] [ 36 ]


Sperm competition risk also influences males' choice of female partners. Men prefer to have as low of a sperm competition risk as possible, and they therefore tend to choose short-term sexual partners who are not in a sexual relationship with other men. [ 3 ]  Women who are perceived as the most desirable short-term sexual partners are those who are not in a committed relationship and who also do not have casual sexual partners, while women who are in a committed long-term relationship are the least desirable partners. [ 37 ]  Following the above, women who are at an intermediate risk of sperm competition, that is women who are not in a long-term relationship but who do engage in short-term mating or have casual sexual partners, are considered intermediate in desirability for short-term sexual partners. [ 37 ]


High levels of sperm competition among the great apes are generally seen among species with polyandrous (multimale) mating systems, while lower rates of competition are seen in species with monogamous or polygynous (multifemale) mating systems. [ 4 ] [ 5 ] [ 38 ]  Humans have intermediate levels of sperm competition, as seen by humans' intermediate relative testis size, ejaculate volume, and sperm midpiece size, compared with other primates. [ 4 ] [ 5 ]  This suggests that there has been a relatively high degree of monogamous behavior throughout our evolutionary history. [ 38 ]  Additionally, the lack of a  baculum  in humans [ 39 ]  suggests a history of monogamous mating systems.


Males have the goal of reducing sperm competition by selecting women who are at low risk for sperm competition as the most ideal mating partners. [ 37 ]


Noticing that sperm in a mixed sample tends to clump together--making it less mobile--and to have a high mortality rate, reproductive biologist Robin Baker, formerly of the University of Manchester, proposed about a decade ago [ when? ]  that some mammals, including humans, manufacture "killer" sperm whose only function is to attack foreign spermatozoa, destroying themselves in the process.


To test this idea, reproductive biologist Harry Moore and evolutionary ecologist Tim Birkhead of the University of Sheffield in the U.K. mixed sperm samples from 15 men in various combinations and checked for how the cells moved, clumped together, or developed abnormal shapes. "These are very simple experiments, but we tried to mimic what goes on in the reproductive tract," Moore says. The team found no excess casualties from any particular donor or other evidence of warring sperm, they report in the 7 December Proceedings of the Royal Society. "The kamikaze sperm hypothesis is probably not a mechanism in human sperm competition," says Birkhead.


The findings are "the nail in the coffin for the kamikaze hypothesis," says Michael Bedford, a reproductive biologist at Cornell University's Weill Medical Center in New York City. He says he had never given the idea much credence.  [ 40 ]


A survey of 67 studies reporting nonpaternity suggests that for men with high paternity confidence rates of nonpaternity are (excluding studies of unknown methodology) typically 1.9%, substantially less than the typical rates of 10% or higher cited by many researchers. [ 41 ]  Cuckolded fathers are rare in human populations. "Media and popular scientific literature often claim that many alleged fathers are being cuckolded into raising children that biologically are not their own," said Maarten Larmuseau of KU Leuven in Belgium. "Surprisingly, the estimated rates within human populations are quite low--around 1 or 2 percent." "Reliable data on contemporary populations that have become available over the last decade, mainly as supplementary results of medical studies, don't support the notion that one in 10 people don't know who their "real" fathers are. The findings suggest that any potential advantage of cheating in order to have children that are perhaps better endowed is offset for the majority of women by the potential costs, the researchers say. Those costs likely include spousal aggression, divorce, or reduced paternal investment by the social partner or his relatives. The observed low cuckoldry rates in contemporary and past human populations challenge clearly the well-known idea that women routinely 'shop around' for good genes by engaging in extra-pair copulations to obtain genetic benefits for their children," Larmuseau said.  [ 42 ]


Women are loyal to men who are good providers. "With DNA tests now widely available, so-called paternity fraud has become a staple of talk shows and TV crime series. Aggrieved men accuse tearful wives who profess their fidelity, only to have their extramarital affairs brought to light...The rule of thumb seems to be that males of higher socioeconomic status, and from more conventionally bourgeois societies, have greater warranted paternity confidence. Lower paternity confidence among those who are the principals for sensational media shouldn’t be surprising then." [ 43 ]


The relative size of  human  male testes is bigger than those primates who have single-male (monogamous or polygynous) mating systems, such as  gorillas  and  orangutans , [ 13 ]  while it is smaller when compared to primates with polyandrous mating systems, such as  bonobos  and  chimpanzees . [ 4 ] [ 5 ] [ 13 ]  While it is possible that the large testis size of some primates could be due to seasonal breeding (and consequently a need to fertilize a large number of females in a short period of time), evidence suggests that primate groups with multi-male mating systems have significantly larger testes than do primates groups with single-male mating systems, regardless of whether that species exhibits seasonal breeding. [ 38 ]  Similarly, primate species with high levels of sperm competition also have larger ejaculate volumes [ 4 ]  and larger sperm midpieces. [ 5 ]


Unlike all other Old World great  apes  and monkeys, humans do not have a baculum (penile bone). Dixson [ 44 ]  demonstrated that increased baculum length is associated with primates who live in dispersed groups, while small bacula are found in primates who live in pairs. Those primates that have multi-male mating systems tend to have bacula that are larger in size, in addition to prolongation of post-ejaculatory intromission and larger relative testis size. [ 21 ] [ 45 ]


Seasonal  variation in human  birth rate  has been found to be a nearly universal phenomenon. [ 1 ]  Also, birth seasonality has been found to be correlated with certain physiological and psychological traits of humans and animals [ 2 ] [ 3 ]  and  type I diabetes . [ 4 ]  Evidence for seasonality in humans is limited. [ 5 ]


The season in which babies are born can have an effect on their future risk of developing neurological disorders like  seasonal affective disorder ,  bipolar depression , and  schizophrenia ; [ 2 ] [ 3 ]  as well as  type I diabetes . [ 4 ]  Research has shown that the season of a baby’s birth can have an effect on whether or not they will become a heavy smoker. Although the aspects of this effect differ by sex, the effect exists across both of them. [ 6 ]