Why we yawn: 5 theories

The question of why humans yawn — what, exactly, does it accomplish? — has long stumped scientists. Last week, experts gathered in Paris to take a sustained look at the problem during the first-ever International Conference on Yawning. While no definitive answers were reached, here are five leading hypotheses:

1. A mechanism for cooling the brain

In 2007, University of Albany researchers Andrew C. Gallup and Gordon G. Gallup concluded that humans yawn in order to lower the temperature of their brains and remain more alert to danger. Similar to a fan in a computer, they speculate, yawning draws in cooler air and optimizes neurological function at a time when people are drowsy and therefore more vulnerable.

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2. An invitation to sex

A partner's yawn might not seem like the most erotic of gestures, but Dutch researcher Wolter Seuntjens believes it is sometimes a subconscious invitation to sex. Seuntjens first developed the theory after hearing anecdotes about people who yawn during intercourse. He and fellow researchers say they still do not understand the relationship between sex and yawning, and cannot decipher between a yawn caused by attraction and one caused by a need to sleep.

3. A means of building empathy

In an attempt to explain yawning's infectious nature, researchers at London University's Birkbeck College, conducted a study of autistic children, who often lack the ability to develop normal emotional ties or feel empathy. They found that, while children with autism still yawn, they are immune to contagious yawning. While the study does not completely answer why animals (from humans to dogs to fish) yawn, it does suggest that contagious yawning is directly tied to emotional empathy and communication.

4. An way to boost oxygen in the body

For many years, the leading hypothesis was that yawns were triggered by low oxygen levels in the lungs. The air exchange from yawning would deliver a much-needed boost oxygen into the bloodstream. The reasonable-sounding explanation even made its way into medical textbooks. But the theory now appears to be flawed: Scientists have found that our lungs cannot detect low levels of oxygen.

5. A leftover, useless behavior from

Some scientists think that we yawn for no reason whatsoever. According to Richard Roberts of Tennessee's Genetics and Prenatal Diagnostic Center, the action could simply be a vestige of our days in the womb, when fetuses yawn to clear their airways and allow fluid to escape. "There is no reason why babies and adults and children need to yawn and hiccup," says Roberts.

Sources: ABC, Telegraph, The Australian, Science Daily, Independent, Scientific American, New Scientist