While it's true after all that yawning really is contagious, that's not the only factor that makes people yawn upon seeing other people yawn. However, scientists still can't put a finger on what other factors induce contagious yawning.

A new study has found that age may have something to do with contagious yawning, but not so much. The researchers, led by Elizabeth Cirulli, assistant professor of medicine at the Duke University Centre, tested 328 healthy adults aged between 18 and 83. Researchers submitted the participants to a test that measured their cognitive performance as well as empathetic traits, such as identifying with fictional characters, feelings of sympathy or distress over another person's misfortune.

Then the participants watched a video showing people yawning. The video, which ran for three minutes, included people of all ages, from babies to centenarians, of various ethnicities and genders, and they were all yawning. The participants were asked to click on a button each time they yawned upon seeing the person in the video yawn.

The results, which were published in PLOS ONE, has shown that on the average, the participants yawned four times during the course of the video they were viewing. About 68 percent - or 222 participants -- yawned at least once. Of that number, 82 percent of people younger than 25 yawned. About 60 percent of people aged from 25 to 49 yawned, while 41 percent of those over 50 yawned.

When 129 of the participants watched the same video a second time, it was found that those who yawned during the first time they watched the video were most likely to yawn again upon the second viewing. This gave the researchers a clue that the factor causing contagious yawning is a consistent and stable trait.

Nevertheless, the verdict remains unclear. There was no strong connection found between yawning and empathy or intelligence. Although frequent yawners scored a little bit higher on empathy and cognitive performance, age seems to emerge as a much stronger influencing factor, since younger participants tended to yawn more, while older participants were least susceptible to yawning.

However, age as a factor was only able to account for eight percent of all contagious yawns. There was something else causing contagious yawns, which the researchers have not found yet.

"Age was the most important predictor of contagious yawning, and even age was not that important," said Cirulli. "The vast majority of variation in the contagious yawning response was just not explained."

The researchers believe that the significance of this study lies in its potential to treat mental disorders. People who suffer from autism and schizophrenia are less likely to catch yawns, and if yawning is inherited, understanding the genes that account for contagious yawning could make way for treatment breakthroughs.

"The contagious aspect of yawning is a well-known phenomenon that exhibits variation in the human population," said the researchers. "Despite the observed variation, few studies have addressed its intra-individual reliability or the factors modulating differences in the susceptibility of healthy volunteers. Due to its obvious biological basis and impairment in diseases like autism and schizophrenia, a better understanding of this trait could lead to novel insights into these conditions and the general biological functioning of humans."

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