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Why does yawning seems to be contagious?
Is this similar to laughter being contagious or does it has a different reason?

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Are you looking for a causative factor (i.e, a neural circuit connects external yawing cues to the internal yawning effector center) or are you interested in the physiological/evolutionary reason (i.e., yawning communicates hypoxic environment to other individuals in the nearby). –  Gianpaolo R Apr 23 '13 at 12:06
    
I would be interested in the physiological/evolutionary reason. –  user1897 Apr 23 '13 at 13:44
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up vote 9 down vote accepted

Yawning is contagious in humans, which means that, in general, it is more likely that a person yawning after perceived (by sight, hearing, or both directions) yawning issued by another person. The frequency of infection varies throughout the day, with a peak in early morning and late evening.

A recent study conducted by Ivan Norscia and Elisabetta Palagi (University of Pisa) has provided the first evidence that the behavioral contagion of yawning is associated with the empathic bond between people. The study showed that the infection follows the same gradient of empathy: is greatest in close relatives (parents / children / grandchildren, brothers, stable couples), decreases in friends, then acquaintances in (people connected only by a third, external, ie work or a friend in common) and reaches a minimum in the unknown.

Various studies of clinical, psychological and neurobiological suggest and support the link between the yawn contagion and empathy. For example, the infection begins to occur at 4-5 years of age, when children develop the ability to identify, in a proper way, the emotions of others. In addition, the infection is reduced or absent in people who have problems related to empathy, how people with autism, and is positively related to subjective scores of empathy based on tests of psychological type. Finally, the areas of the brain related to the emotional sphere overlap, in part, with those involved in the response to yawn, with a possible involvement of mirror neurons.

Yawning has been observed among various primates. In these cases the yawn is a threat gesture, a way of maintaining order in the primates' social structure. Specific studies were conducted on chimpanzees and stumptail macaques. A group of these animals was shown a video of other conspecifics yawning; both species yawned as well. This helps to partly confirm a yawn's "contagiousness".

In animals, yawning can serve as a warning signal. For example, Charles Darwin, in his book The Expression of the Emotions in Man and Animals, mentioned that baboons yawn to threaten their enemies, possibly by displaying large canine teeth.

see yawn

autism

Gordon Gallup, who hypothesizes that yawning may be a means of keeping the brain cool, also hypothesizes that "contagious" yawning may be a survival instinct inherited from our evolutionary past. "During human evolutionary history, when we were subject to predation and attacks by other groups, if everybody yawns in response to seeing someone yawn the whole group becomes much more vigilant and much better at being able to detect danger.

Personally, I can make an observation. Cats yawn as specific means to calm people, and especially to reassure the other cats and the pups. In order, they stretch, yawn, lie down, half close theyr eyes as if they start to sleep. When the whole group repeats the gestures, the majority of them (now calm) actually falls asleep.

EDIT @kmm

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Could you add some links to the studies you cite (Norscia & Palagi; Gallup) so that interested parties can follow up with the primary literature? –  kmm Apr 25 '13 at 22:57
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