Dear Doctor: The 7-year-old in our family has discovered yawns are "catching," and he just loves it when he can get his daddy to start yawning, too. Of course, now he wants to know why, but even after looking online, we're not sure of the answer. Does anyone really understand yawning?

Dear Reader: True confession -- reading your question triggered a yawn. (OK, two.) That's not unusual, as it's estimated that for well over half of us, yawns are contagious. In a study at Duke University, a video of people yawning had close to 70 percent of the 328 participants doing the same during the three-minute showing, some as many as 15 times. Lest we think this is a humans-only phenomenon, contagious yawning is also a hallmark of chimpanzees and a group of primates known as Old World monkeys. And as dog owners can attest (yes, there are studies into this as well), it's a trait also shared by many of our canine companions.

So why do we yawn and why are they catching? Though these questions have tantalized scientists, philosophers and all of us yawners for millennia, we're still short on definitive answers. As far back as 400 B.C., Hippocrates pondered the origins of the spontaneous yawn. That is, a yawn that occurs without the prompt of someone else's yawn. He associated it with a general reflex to cool the body, which turns out to be a decent guess. Other theories put forth to explain the spontaneous yawn have included drowsiness, boredom, weariness and empathy. More recently, researchers have come to see potential for the yawn to be used as a diagnostic marker of neurological disease. To that end, there's now a specially designed yawning susceptibility scale to measure exactly how prone someone is to "catching" a yawn.

In recent years, researchers have identified a link between temperature and yawning, thus giving Hippocrates' theory from 2,000-plus years ago a nice boost. In one study, 120 pedestrians selected at random during both hot and cold weather were found to "catch" a yawn more frequently within a certain window of warmer temperatures. In another experiment, researchers were able to affect the rate of both spontaneous and contagious yawning with the use of cold and hot packs. Variables like the person's sex or age, how much they had slept the night before, time they spent outdoors, humidity and the season of the year didn't influence their yawning behavior.

Last year, researchers in England found a connection between spontaneous yawning and the primary motor cortex, a region of the brain that takes a lead role in generating the messages that initiate our physical movement. They also discovered that trying not to yawn actually increases the sense of needing to yawn. According to the researchers, these findings may help shine a light on conditions associated with impulse control, such as obsessive-compulsive disorder or Tourette's syndrome.

Thus far, research into yawning has added tantalizing bits and pieces of information about a simple action with complex origins. That's why, despite your best efforts at research, a definitive answer eluded you. Chances are, though, you yawned while reading this column. And if he was in the room with you, so did your 7-year-old.

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