Why Yawn?

To paraphrase a famous Cole Porter song, birds do it, chimpanzees in the zoos do it and some courageous kangaroos do it.

It's not love we're speaking of, but something we all do: yawning.

It's contagious! And lots of theories attempt to explain not only why it's contagious but why we yawn at all. Hippocrates, a physician in the fourth century B.C. believed it expelled bad air while increasing good air in the brain.

That may be why one of today's theories posits that yawning increases blood oxygen levels while decreasing carbon dioxide levels. Yet, so far no science has proven this link.

Another theory is that yawning plays a role in regulating brain physiological processes such as trying to stay awake.

Actually, yawning does nothing to keep us more alert. Then there's the thermoregulation hypothesis, which says yawning affects brain temperature, but the supporting data is a stretch.

If there's no obvious physiological reason for yawning, could there be a social one?
Across cultures, yawning is interpreted as a sign of boredom or fatigue, so the hypothesis is, it's a form of non-verbal communication that synchronizes the behavior of a group.

As we saw when we started the episode, yawning is contagious. However, it is only contagious in people with intact social competence. For example, people with autism or schizophrenia are not as susceptible.

What about animals? Chimpanzees are susceptible to contagious yawning while lions are not, and dogs vary.

However, yawning appears to be much more complex because - don't forget - people and animals also yawn when alone.