why are yawns contagoius?
367895
I don't think there is a scientifically proven answer, but I like the theory that it is genetic.  Showing an open mouth and teeth is a form of aggression in many animals, including chimpanzees (closest genetic code to homo sapiens).  Many times when a chimp shows its teeth to another chimp they do it back to each other.  Theory goes that when someone yawns, you yawn back because of some gentic vestige left over from when we were monkeys.  They are being aggressive and you are defending your turf.
422922
It is also believed to be linked to social bonding.\n\nExperiments with chimps have shown that they also yawn when another chimp yawns.  Also videos (night video) in caves, where there is no light, where a certain species of monkey sleep - shows that they all yawn audibly before going to sleep.  Kind of a way of letting everyone know that they're there, even though they can't see each other.\n\nThis has probably carried through to humans.\n\nHave a look at http://www.theanswerbank.co.uk/Science/Question117042.html\n\nor\nhttp://webperso.easyconnect.fr/baillement/contagion-platek.html
40340
An indication of tiredness, stress, over-work or boredom. \nAn action indicating psychological decompression after a state of high alert. \nA means of expressing powerful emotions like anger, rejection or apathy. \n\nYou see someone yawning.  You hear someone yawning.  You read about yawning. You think about yawning. \nNow, you want to yawn. If this describes you, then you have just "caught" a yawn. Between 40 and 60% of the population is susceptible to contagious yawns, but the reason why yawns are catching is unknown. Steven Platek, Ph.D. suggests that being aware of one's own mental state ("self-recognition") and the ability to see things from another person's point of view ("mental state attribution") may make people susceptible to contagious yawns.
775112
It's a sympathetic reaction that makes them contagious.  A yawn happens when your body is not receiving enough oxygen.  You yawn when someone else yawns because of this sympathetic reaction.  Mythbusters did an experiment on this topic on one of their shows.
239646
It's contagious to only those with an empathetic mind. Not everyone has one, so not everyone is going to yawn if someone else does.\n\nDon't listen to that Brisbane Roo character--he's full of it. It's true what he said about periods, but that has nothing to do with yawning.
196592
It is the brain requiring more oxygen. Which is why when someone else yawns you do to, it is your brain making sure that it has enough in case the other person takes it all. Not very likely to happen unless in a confined space, but the brain is always on survival mode.
198710
When you yawn, you are attempting to take in as much air as possible, more oxygen gives more energy.  People in the immediate area are in competition for this air too and yawn as a response to retrieve air themselves
330313
This is science.\n\nWhen you yawn, it is because your brain lacks oxygen and need to get more oxygen. This is done by yawning where you inhale deeply to take in more air and oxygen. \n\nWhat you mentioned is especially true, especially in crowded places or enclosed palces.\n\nWhen you yawn, you take in large amount of air from the surrounding, depriving the other of the oxygen. Thus, the others will need more oxygen too. So, they yawn to take in more air. And when they yawn, they take in even more air from the surrounding and the others will have to yawn to get sufficient oxygen.\n\nOne yawn will spark off a chain reaction of yawning.\n\nUnderstand now?
261027
Why is this the second most asked question on here?  If the guys would stop asking about the size of their "manhood", this would be number one!\n\nWhy we yawn when we see others yawning has baffled scientists for ages and a number of suggestions have been put forward to explain this phenomenon. What we do know is that yawning is an involuntary action- our brain does it without us thinking about it, even very young babies yawn showing that it's a built in action (Chimps do it too by the way). Just thinking about yawning can trigger the bit of the brain that causes a yawn. http://www.ratlab.co.uk/yawn.htm...\n\nMore theories can be found at the links below: \n\nhttp://www.world-science.net/exclusives/...\nhttp://www.yesmag.bc.ca/questions/yawn.h...
295331
Contagious is the wrong word. \nBut they are a suggestion, almost hypnotic, to yawn.\nAs is the word "itch", just the talk of creepy crawlies and itches is enough to start you itching somewhere on your body.\nHope this helps
289289
Between 40 and 60% of the population is susceptible to contagious yawns, but the reason why yawns are catching is unknown.\n\nI yawned once when typing up this answer.
1002474
The yawn reflex is often described as contagious: if one person yawns, this will cause another person to "sympathetically" yawn.[2] The reasons for this are unclear; however, recent research suggests that yawning might be a herd instinct.[3] Other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack during a full moon. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. It can serve as a warning in displaying large, canine teeth. This phenomenon has been observed among various primates. The threat gesture is a way of maintaining order in the primates' social structure. The contagion of yawning is interspecific (i.e., try yawning in front of your dog). Yawning in public is generally regarded as impolite in the West, but came into fashion in polite French society for a brief period in the late 18th century. Oddly, sometimes sympathetic yawning may be caused by simply looking at a picture of a person or animal yawning, or even seeing the word "yawn".
204275
Yawning or "pandiculation," is still pretty much a mystery to psychologists. (I just asked a friend in the trade.) Aside from the obvious observations of how it's related to boredom and how it gets "contagious" after a certain period of development in people, there isn't that much information on it. Sorry. (Hey, this is info from an old friend, so you might want to check newer sources.)
632459
Causes of yawning\n1.An indication of tiredness, stress, over-work or boredom. \n2.An action indicating psychological decompression after a state of high alert. \n3.A means of expressing powerful emotions like anger, rejection or apathy. \n4.A sign that one is not breathing deeply, and not receiving enough oxygen through the body. \nA yawn can express strong anti-social messages, and so in some cultures people try to mute or mask them by placing a concealing hand over the yawning mouth.\n\nA long-standing hypothesis is that yawning is caused by an excess of carbon dioxide and lack of oxygen in the blood. The brain stem detects this and triggers the yawn reflex. The mouth stretches wide and the lungs inhale deeply, bringing oxygen into the lungs and hence to the bloodstream. It is almost certain however, that this hypothesis is not correct. One study documented that this effect does not exist ("Yawning" by Robert R. Provine, pages 532-539, American Scientist, November-December 2005, Vol 93, No. 6). A more recent hypothesis is that yawning is used for regulation of body temperature. Another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that affect emotions, mood, appetite and other phenomena. These chemicals include serotonin, dopamine, glutamic acid and nitric oxide. As more of these compounds are activated in the brain, the frequency of yawning increases. Conversely, a greater presence in the brain of opiate neurotransmitters such as endorphins, reduces the frequency of yawning. Patients taking the serotonin reuptake inhibitor Paxil (Paroxetine HCl) have been observed yawning abnormally often.\n\nAnother theory is that yawning is similar to stretching. Yawning, like stretching, increases blood pressure and heart rate while also flexing many muscles and joints. Some have observed that if you try to stifle or prevent a yawn by clenching your jaws shut, the yawn is unsatisfying. As such, the stretching of jaw and face muscles seems to be necessary for a good yawn.\n\nThe yawn reflex is often described as contagious: if one person yawns, this will cause another person to "sympathetically" yawn.[2] The reasons for this are unclear; however, recent research suggests that yawning might be a herd instinct.[3] Other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack during a full moon. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. It can serve as a warning in displaying large, canine teeth, thus proclaiming, "don't attack while I am sleeping..." The contagion of yawning is interspecific (i.e., try yawning in front of your dog). Yawning in public is generally regarded as impolite in the West, but came into fashion in polite French society for a brief period in the late 18th century. Oddly, sometimes sympathetic yawning may be caused by simply looking at a picture of a person or animal yawning, or even seeing the word "yawn".\n\nAdelie Penguins employ yawning as part of their courtship ritual. Penguin couples face off and the males engage in what is described as an "ecstatic display," their beaks open wide and their faces pointed skyward.
687663
yawning is contagious because you watch the other person yawning.  However, over the phone it is not contagious.  Also, if you are sitting on the same room but do not see the other person yawning then it contagious.  It is a way of reciprocating unconsciously.
65932
From straightdope:\n\n"nobody really knows. Fact is, we don't know why people yawn, period.\n\nIt was long believed you yawned when there was too much carbon dioxide and not enough oxygen in your blood. A part of your brain called the brain stem detected this and triggered the yawn reflex. Your mouth stretched wide and you inhaled deeply, shooting a jolt of oxygen into the lungs and thence to the bloodstream.\n\nSubsequently, you exhaled a lot of CO2. Often you'd stretch while yawning, which seemed to temporarily improve circulation. You yawned and stretched a lot more when you got tired because your breathing slowed down.\n\nOr so people thought. In recent years, though, a few radicals have said the preceding is all malarkey. Who knows, they say, maybe we yawn because it's too warm in the room.\n\nCecil isn't about to settle the issue here, and he doesn't need to. We merely observe that whatever yawn-inducing conditions prevail for you also apply to your friends.\n\nIf you're out late in some crowded dive, you're probably all tired, all warm under the collar, and all breathing the same stale air. You're probably all on the verge of a yawn, too, and the power of suggestion from seeing one person do it is enough to push everybody else over the edge.\n\nAdults rarely catch a case of the yawns from a child or animal, which tends to corroborate this idea.\n\nChildren usually have different sleep schedules and respiration rates from adults, so you would expect them to yawn at different times. Animals, on the other hand, often yawn not for physiological reasons but as a display of hostility, to which humans are evidently unresponsive."
1170939
From what I've read, we are not sure exactly why that happens, but many doctors and scientist believe is is due to a mental reaction that sees a person yawn and believes you should do so as well.  Yawning is a response to a lack of oxygen in the brain under idle conditions, such as standing around or sitting in a chair.  So when you see someone yawn, your brain triggers a yawn to make sure you are also not starving your brain.