One of science's most baffling questions? Why we yawn
Provine tells me this often happens when people are talking to him; during presentations, he sometimes finds the majority of his audience with their mouths agape and tonsils swinging. Luckily, as a psychologist at the University of Maryland, Baltimore County and author of Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond, he isn't offended. "It makes a very effective lecture," he says. "You talk and then the audience starts yawning. And then you can ask people to experiment on their yawns - like closing the lips, or inhaling through clenched teeth, or trying to yawn with the nose pinched closed." It is through experiments like these that Provine has tried to explore a millennia-old mystery: why do we yawn? We all know that tiredness, boredom, or the sight of someone else can all bring along the almost irrepressible urge - but what purpose does it serve the body? When he first started work on so-called "chasmology" in the late 80s, Provine wrote that "yawning may have the dubious distinction of being the least understood, common human behaviour". Nearly three decades later, we may be closer to an answer, but it's one that has split the field. Arguably the first studier of yawns was the Greek physician Hippocratesnearly 2,500 years ago. He believed that yawning helped to release noxious air, particularly during a fever. "Like the large quantities of steam that escape from cauldrons when water boils, the accumulated air in the body is violently expelled through the mouth when the body temperature rises," he wrote. Different incarnations of the idea lingered until the 19th Century, when scientists instead proposed that yawning aids respiration - triggering a rush of oxygen into the blood supply, while flushing out the carbon dioxide. If that were true, you would expect people to yawn more or less frequently depending on the oxygen and carbon dioxide concentrations in the air. Yet when Provine asked volunteers to breathe various mixtures of gases, he found no such change, BBC News reports. Many theories have instead focussed on the strange, contagious nature of yawning - a fact that I know only too well from my conversation with Provine. "Around 50% of people who observe a yawn will yawn in response," he says. "It is so contagious that anything associated with it will trigger one... seeing or hearing another person, or even reading about yawning." For this reason, some researchers have wondered if yawning might be a primitive form of communication - if so, what information is it transmitting? We often feel tired when we yawn, so one idea is that it helps set everyone's biological clocks to the same rhythm. "In my view the most likely signalling role of yawning is to help to synchronize the behaviour of a social group - to make them go to sleep more or less at the same time," says Christian Hess, at the University of Bern in Switzerland. With the same routine, a group can then work together more efficiently throughout the day.
