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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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The mystery of why we yawn

It's contagious and seems to serve no physiological purpose

My latest Mind and Matter column in the Wall Street Journal is on yawning:

Even as scientists get better at finding explanations for animal behavior-at the genetic, physiological, evolutionary and neural level-certain habits remain implacably mysterious. And this is true even when we're the species in question and can see the behavior from the inside. Yawning, for instance: You might think that we would know why we yawn, but it has no obvious need, function or effect.

A recent study confirmed that babies who open their mouths in the womb are indeed yawning, not just gaping. This was long suspected and may help to nail shut the coffin of a theory that has refused to die: that yawning is about filling the lungs with oxygen or emptying them of carbon dioxide.

It's now 25 years since Dr. Robert Provine, now at the University of Maryland, did the obvious experiment. He enriched the oxygen or the carbon dioxide in the air breathed by experimental subjects and found not a hint of suppression or exacerbation of yawning compared with control subjects breathing normal air.

So what is known about yawning? Dr. Provine-who is a champion of what he calls "sidewalk neuroscience," experiments anybody can do at home without special equipment-has spent years teasing out the details of yawning, as recounted in his recent book "Curious Behavior: Yawning, Laughing, Hiccupping and Beyond." By asking people to pinch their noses or grit their teeth while yawning he found that "the motor program…will not run to completion" unless you can inhale through your mouth and gape your jaw wide.

Experiments by Andrew Gallup at Princeton University, his father Gordon Gallup and a colleague found that yawning is suppressed by a cool pack strapped to the forehead or by summer temperatures that are higher then body temperature, and that therefore the purpose of yawning may be to cool the brain by inhaling air. But then why do fetuses yawn and why does the need to yawn not get satiated? Having yawned, you often yawn again; it comes in bouts.

Folklore is correct that yawning is triggered by boredom, drowsiness, stretching or other people yawning. Yawning is so contagious and suggestible that even reading an article about it can trigger the reflex, though the contagion is unconscious: It isn't easy to yawn to order. Nor is such contagion confined to human beings; it has been found in baboons, perhaps, and chimpanzees, for sure.

If yawning is empathetic, Matthew Campbell and Frans de Waal of Emory University in Atlanta predicted that chimpanzees would catch yawning from their "in-group" friends but not from "out-group" strangers. Sure enough, 23 chimps yawned more while watching videos of their in-group yawning than when watching videos of strange apes yawning (or familiar apes not yawning).

Yawning can be triggered by the hormone oxytocin-which is released in the brain during empathetic actions like touching, kissing or cooperating, and which triggers a release of dopamine, a feel-good neurochemical. Fabrizio Sanna and colleagues at the University of Cagliari in Italy injected oxytocin into particular parts of the brains of rats and induced yawning, or blocked the effect by injecting an oxytocin-blocking chemical first.

But this is where introspection lets us down. Contagious yawning doesn't feel much like collaborative social bonding; it just feels like involuntary emulation with little emotional baggage. Nor does empathy suggest a physiological "reason" for yawning (after all, we yawn when alone), though Dr. Provine thinks that it may be "a response to and facilitator of change in behavioral or physiological state," maybe synchronizing a group of people about to embark on a behavioral transition. There's a persistent anecdote that parachutists yawn just before jumping out of airplanes.

Perhaps the main purpose of yawning is to remind us how mysterious human beings still are, even to themselves.