The yawn

By the end of this paragraph, some readers will already be doing it. Yawns spread by sight, by sound, by thought, and by print; they begin in the womb around twenty weeks; and politely covering your mouth does nothing to stop one from arriving. Whatever a yawn is for, it is extremely good at making more of itself.

The oldest explanation is the deadest. Everyone knows a yawn tops up your oxygen, and everyone is wrong: in lab tests, breathing extra oxygen or extra carbon dioxide does not change how much people yawn. The air theory survives because it sounds like it should be true, which is not how air works.

In 2007, Andrew and Gordon Gallup tried temperature instead. Volunteers watched videos of people yawning, a reliable trigger: under ordinary conditions about 48 percent of them caught a yawn. Told to breathe only through their noses, not one person did. In a second round, a warm pack held to the forehead left the rate at 41 percent, while a cold pack dropped it to 9. One working explanation: a yawn is a radiator. The deep inhale and the jaw stretch push cooler air and blood past a warm brain, and anything that cools the brain another way takes the edge off the urge.

In 2003, Steven Platek and colleagues tried personality instead. People who caught yawns scored higher on recognizing their own face and on spotting social blunders in short stories, and lower on traits that blur the line between self and other. Contagious yawning does not show up in children until the early school years, and at the time had been demonstrated convincingly only in humans and chimpanzees. One working explanation: a yawn is a mirror. Catching one rides the same machinery that models other minds, which is why it spreads between people and not from screens showing objects.

Then in 2014 a Duke team ran the collision experiment: 328 people, the same yawning video, and a battery measuring everything the theories care about. Susceptibility turned out to be a stable personal trait - test the same person twice and you get the same answer, and the people who did not yawn mostly did not even feel the urge. But empathy, intelligence, and time of day all washed out once age was accounted for, and age itself explained only 8 percent of the differences. The trait is real, durable, and mostly unexplained.

Which leaves a strange, honest picture. Two theories with real experimental numbers, one heritable-seeming trait that refuses to be assigned a cause, and a stimulus you have been marinating in for four paragraphs. If you have yawned, you have joined slightly less than half of everyone tested. Whether your brain just wanted cooling or company, the literature cannot yet tell you. It can only tell you that you did.