Why when someone that sees a person yawn,they start yawning?
397407
Though the other answers are right (that yawning occurs to signal tiredness when the brain demands extra oxygen due to slipping performance), I would like to add another related comment.\n\nI recently read in the newspaper that scientists are discovering that yawning is a signal of compassion.  Researchers conducted studies in which participants watched videos of people yawning and the researchers measured the time between the yawn of the person on the video and the ensuing yawn of the study participant.  They then somehow effectively linked this elapse to compassion; what they stated was that the quicker you yawn after you see someone else yawn, the the more compassionate you are.\n\nRegretfully, I don't know the web address of the article, which I found intriguing, but I think it is worth your time to investigate.
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.
441827
See the article in the site below.  It tells you why we yawn and gives evidence to support the theory that yawning is "contagious" because "as social creatures we are highly inclined to copy one another."\n\nThe article starts by saying that a good trick to see if anyone is checking you out is to yawn and see if anyone else yawns back!  \n\nHope this helps...
246554
Yes. The mere suggestion of yawning can induce a yawn in some people. (In fact I'm yawning right now from reading your question.) So if you sent an email saying "I'm yawning" to such a sensitive person, even if (s)he couldn't see or hear you that person could yawn.\nAlso, two people might yawn excessively if they shared an environment that is conducive to yawning, i.e., low in oxygen or high in carbon dioxide, but that wouldn't really be contagion.
17567
Yawning is one of those bodily mysteries that scientists haven't completely figured out yet.  The best theory I could find was that you yawn because you're not getting enough oxygen to the brain (maybe there is a lack of oxygen in the space you're standing).  Whenever you're aware of someone yawning, it activates a reaction in your subconscious mind that acts as a threat response.  Basically, whenever you see someone yawn, it makes you body think there is a shortage of oxygen too, and you yawn in response.  No one knows if this is actually the case, but I think that it is a good theory.
604697
lolol...yawns are contagious..u just made me yawn reading ur question...brat!!!
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?
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.
830
monkey-see monkey-do. Dr. Steven M. Platek has said that we yawn b/c we are unconciously imitating other humans. (probably some kind of evolution thing) I guess there are many other species that yawn...fish, cats...(i would totally pay to see a fish yawn!).\nBy the way although it used to be believed that it was to get more oxygen, this, however, is not true.
339257
Yeah, the previous answers are right - lack of oxygen to the brain.\n\nBut the big question is why are yawns contagious? Even experts don't know that.
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
414151
What's behind this mysterious epidemic of yawning? First, let's look at what a yawn is. Yawning is an involuntary action that causes us to open our mouths wide and breathe in deeply. We know it's involuntary because we do it even before we are born. Research shows that 11-week-old fetuses yawn. \n\nThere are many parts of the body that are in action when you yawn. First, your mouth opens and jaw drops, allowing as much air to be taken in as possible. When you inhale, the air taken in is filling your lungs. Your abdominal muscles flex and your diaphragm is pushed down. The air you breath in expands the lungs to capacity and then some of the air is blown back out. \n\nWhile the dictionary tells us that yawning is caused by being fatigued, drowsy or bored, scientists are discovering that there is more to yawning than what most people think. Not much is known about why we yawn or if it serves any useful function, and very little research has been done on the subject. However, there are several theories about why we yawn. Here are the three most common theories: \n\n\nThe Physiological Theory -- Our bodies induce yawning to drawn in more oxygen or remove a build-up of carbon dioxide. This theory helps explain why we yawn in groups. Larger groups produce more carbon dioxide, which means our bodies would act to draw in more oxygen and get rid of the excess carbon dioxide. However, if our bodies make us yawn to drawn in needed oxygen, wouldn't we yawn during exercise? Robert Provine, a psychologist at the University of Maryland, Baltimore County, and a leading expert on yawning, has tested this theory. Giving people additional oxygen didn't decrease yawning and decreasing the amount of carbon dioxide in a subject's environment also didn't prevent yawning. \nThe Evolution Theory -- Some think that yawning is something that began with our ancestors, who used yawning to show their teeth and intimidate others. An offshoot of this theory is the idea that yawning developed from early man as a signal for us to change activities. \nThe Boredom Theory -- In the dictionary, yawning is said to be caused by boredom, fatigue or drowsiness. Although we do tend to yawn when bored or tired, this theory doesn't explain why Olympic athletes yawn right before they compete in their event. It's doubtful that they are bored with the world watching them. \nThe simple truth is that even though humans have been yawning for possibly as long as they have existed, we have no clue as to why we do it. Maybe it serves some healthful purpose. It does cause us to draw in more air and our hearts to race faster than normal, but so does exercise. There's still much we don't understand about our own brains, so maybe yawning is triggered by some area of the brain we have yet to discover. We do know that yawning is not limited to man. Cats, dogs, even fish yawn, which leads us back to the idea that yawning is some form of communication. \n\nHave we provoked a yawn out of you yet? If we have, hopefully it's not out of boredom, but by the power of suggestion.
522649
Prehistoric hunter gatherers had to face many threats, from big predators to neighbouring tribes.\n\nOne theory (and it's just a theory, mind, no one knows for sure) is that yawning might have evolved as a way of drawing attention to the fact that the guy on watch was getting sleepy.\n\nThe guard yawns involuntarily, the other hunters in the group might not notice it consciously, but subconsciously they do, and they pass it on. Eventually someone notices that the group is tired and goes to checks on the the guy on watch.\n\nI like this theory, it makes sense to me.
317540
A Yawn is the brain's natural way of rapidly increasing the amount of oxygen taken into the lungs for use by the body during times of fatigue. It is triggered by very delicate mental cues and accordingly, can be triggered by seeing someone else yawn or even thinking about yawning long enough.
41848
I have a theory on this to share.\nMay be yawning is not a chain reaction at all.\nIts just our reaction to the state of things, outside as well as inside our body.\nFor instance, we feel sleepy when our brains are tired or refuse to process. Result is that we all yawn when after a tired game, after a boring speech.\nMay be even when there is lack of sufficient oxygen.\nWe all have different reaction times though. Thats probably the reason why its being mistaken to be a chain reaction.
680125
Here's a theory. Yawning can be caused by lack of oxygen, and if a bunch of people are in the same room which is poorly ventilated (no windows open, no air conditioning, or it's a stiflingly hot room) they could all yawn because of this, as they are all in the same place.\n\nEither that or they are all tired! :D
309172
padillio is right. it is just a mental thing, and she is also right in saying that it is because of a lack of oxygen. it is also associated with being tired b/c when you're body is fatigued or tired, you're breathing slows down and you get a lack of oxygen. yawning is an involuntary muscle movement to make you breath heavily and take in more oxygen. anyway, that's pretty much it.