Call it the finding that puts the "aw" in yawn—scientists have discovered that people yawn more in response to the yawns of people they care about most.

So-called contagious yawning is a type of psychological effect that happens only in response to seeing, hearing, or reading about yawning.

Though there have been previous hints of a link between contagious yawning and empathy, the new study marks the first time scientists have actually observed the connection.

New statistical models showed that the rate at which people yawned contagiously was highest in response to kin, then friends, then acquaintances, and lastly strangers.

The findings suggest that yawning is a form of empathizing with people experiencing a feeling, which—in the case of yawning—usually means stress, anxiety, boredom, or fatigue.

"This is the important point: By reenacting the mechanism, it's like you share emotions, so your response is higher because you mirrored each other's emotions," said study co-author Ivan Norscia of the Natural History Museum at the University of Pisa in Italy.

By contrast, spontaneous yawning, a purely physiological phenomenon, may occur to cool our brains, according to recent findings.

The Stronger the Bond, the More You Yawn

For the study, Norscia and co-author Elisabetta Palagi, of the Institute of Cognitive Sciences and Technologies in Rome, spent a year collecting behavioral data from more than a hundred adults of different nationalities.

The observations were made in a variety of natural settings—such as on the train or around the dinner table—in Italy and Madagascar, where the study authors also conduct primate research.

The scientists recorded several variables, such as the subjects' relationships to each other, countries of origin, genders, and styles of yawning, i.e., open-mouth versus suppressed yawning.

The team then developed a statistical model based on their data and tested the effects of each variable on contagious yawning.

In the model, only social bonding emerged as a predictor of response to another person's yawn, according to the study, published December 7 in the journal PLoS ONE.

The research provides "a pretty compelling case that empathy may be involved in contagious yawning among humans," said Andrew Gallup, an evolutionary biologist at Princeton University who has also studied yawning.

Are Yawners Paying Attention?

However, Gallup pointed out a potential limitation in the new results: People may simply notice yawning more often in their loved ones and friends.

For instance, people may be more attuned to the behaviors of people they know than those of, say, strangers on a train.

"I don't think that this completely discredits the results, but I think it's something to take into consideration," Gallup said.

A potential remedy, he said, would be to set up an experiment in which people are shown video clips of other people of varying relatedness to the subject, with instructions to pay equal attention to each clip.

If the same connection between social bonds and yawning emerges, "then you're home free," he said.

But study co-author Norscia pointed out that a person may actually pay less attention to a family member's yawn.

That's because a person's response to new stimuli is usually heightened, whereas a person's reaction to familiar stimuli might be reduced.

This is "an evolutionary adaptation to avoid an unbearable overloading of the attentional system," Norscia said. When someone does notice a loved one's yawn, though, they're more likely to respond due to their emotional attachment.

As for the potential study design with video clips, in Norscia's opinion, "I doubt you can drive someone's attention simply by asking them to do it. I wish controlling attention was that easy."

Contagious Yawning Still a Mystery

If people do yawn as a form of empathy, the reasons for the response are still unclear.

Evolutionarily, contagious yawning is a much more recent phenomenon seen only in humans, chimpanzees, baboons, and—more controversially—dogs.

Spontaneous yawning, on the other hand, has been around for at least 200 million years—even fish do it.

A potential benefit of contagious yawning could be to improve the overall vigilance of a group, Princeton's Gallup said. That's because, by cooling the brain, yawning enhances alertness and overall mental efficiency.

Likewise, yawning may be a way of encouraging people to lend a helping hand, study co-author Norscia suggested.