DURHAM — What causes contagious yawning remains unexplained, which actually may be kind of exciting.

Previous studies had suggested that people yawned at the sight of someone else yawning out of empathy. But new research from the Duke University School of Medicine’s Center for Human Genome Variation found that it doesn’t appear to have a strong relationship with empathy, fatigue, intelligence or several other variables.

The study, which appeared Friday in the online research journal PLOS ONE, is believed to be the most comprehensive ever done on the potential influences on contagious yawning.

Yawns come in two basic types: the spontaneous ones set off by such things as fatigue and boredom, and the contagious version triggered by seeing someone else yawn.

So how does a scientist get funding for a yawn study? Elizabeth Cirulli, senior author of the Duke paper, hears that question all the time from friends and colleagues.

Cirulli, an assistant professor of medicine at the genome center, studies the genetic basis for human characteristics. With the yawn study, she was trying to get at some serious and fundamental questions about genetics and human biology.

“There is a really large category of traits that are largely being ignored from the genetic point of view, and I don’t think that they’re useless traits,” she said. “It may seem a little frivolous on the surface – how are you really going to help people by figuring this out? – but I think it’s still important to figure out the basic biology behind why people have certain behaviors or why they have certain differences between them.”

And if while doing that you can figure out a connection to a disease, that could give you a novel viewpoint into how that disease works, she said.

Contagious yawning, for example, has an as-yet unexplained relationship with autism and schizophrenia – people with those disorders are less likely to exhibit the behavior – and Cirulli thinks it could be an endophenotype for them.

Endophenotypes are genetically based traits that correlate with a given illness. Identifying them can help with diagnoses, and understanding them more deeply may also advance the understanding of a particular disease.

Before the researchers could begin the yawn study, they had to create a proper trigger. So Alex J. Bartholomew, then an undergraduate student and now a scientist at the genome center, searched YouTube for images of people yawning. He then spent hours stitching together a 3-minute video.

Bartholomew, who is the first author of the research paper, yawned nonstop the whole time and each time he had to watch the video afterward. Cirulli always found that funny, though the film always made her yawn, too.

“You’d think that the more you watch the video, the more you would get used to it and it would stop making you want to yawn,” she said. “But we have never experienced that.”

Because one thing that significantly suppresses contagious yawns is being watched or videotaped, the subjects of the student were asked to record their yawns themselves. Of the 328 healthy volunteers who watched the video, 222 contagiously yawned, doing so from once to 15 times each. The findings were verified with several testing sessions.

The participants, who were also being evaluated for Cirulli’s larger studies, also took tests on mental function, a demographic survey and a questionnaire that tested levels of empathy, energy and sleepiness.

Unlike the results of earlier studies, the Duke scientists did not find meaningful connections between contagious yawning and empathy, intelligence or time of day. Older subjects were slightly less susceptible to “catching” yawns from others, but that explained just 8 percent of the variability in the responses to the video.

That lack of correlations for the various independent factors suggests a genetic cause, Cerulli said. The next step is for the scientists to delve into genetic data from the subjects. If they find something in the genes that makes people less susceptible to contagious yawns, it’s possible that the variant or variants involved may also be associated with schizophrenia or autism, Cirulli said.

“Even if no association with a disease is found, a better understanding of the biology behind contagious yawning can inform us about the pathways involved in these conditions,” she said.

The yawn study is only a tiny part of the four-year grant from the National Institute of Mental Health that Cirulli works under, she said. It pays for $50,000 in research annually and most of her salary, and is mainly used for her work in seeking links between genetics and intelligence. Part of the study was also funded by Ellison Medical Foundation New Scholar award.

And the other type of yawning, the spontaneous kind? No one really knows the biological reason for that kind, either. But that’s some other researcher’s problem.

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