Italian scientists who documented humans interacting in everyday situations have found that women are more susceptible to catching the urge to yawn from others than men are.
The findings, published in the journal Royal Society Open Science, may shed light on this social contagion and on the differences in empathetic capacity between women and men.
A yawn, which lasts 6 seconds on average, is an involuntary action characterized by opening the mouth, breathing in, expanding a tube that leads to the eardrums and then exhaling. It may be one of the best-known social contagions -- watch a friend yawn, and you might soon find yourself doing the same, even if you weren't sleepy.
"Yawning is contagious in that it can be triggered by others' yawns," the study authors wrote. "A wide range of sensory modes are vectors of contagious yawning in humans, ranging from hearing, seeing, reading about or even thinking about yawning." (This reporter can attest to the action being triggered by reading [and writing] about it.)
There's been building evidence that a yawn's ability to spread to others is somehow correlated with the ability to empathize -- to pick up on others' emotional states and imagine what they may be thinking or feeling.
Contagious yawning seems to increase starting around the age of 4 or 5 years, about the same time children start developing the ability to identify other people's emotions, and it falls when those empathetic abilities also fall in old age. It's also more common between friends and family -- people with whom you share a close personal bond -- than it is between two strangers.
"There is growing evidence that yawn contagion is an empathy-based phenomenon," the authors wrote. "Contagious yawning recruits different neuronal networks involved in empathic processing, including the inferior frontal gyrus and other mirror neuron areas."
If contagious yawning has a link with empathy, then is there a difference in susceptibility between men and women? After all, research has suggested that women demonstrate a greater capacity for empathy than men do.
"If women are more empathic than men, then we also expect that in the susceptible population women are infected at higher rates by others' yawns compared with men," the authors wrote.
To find out, scientists in Italy observed human subjects in their "natural environment" -- at the office, eating dinner and at various social events, said study coauthor Ivan Norscia, a sociobiologist at the University of Pisa.
Over a period of nearly five years (Oct. 26, 2010 to April 27, 2015) they recorded 1,461 bouts of yawning. They restricted their analysis to 92 pairs of people (one trigger and one responder) who experienced at least three separate instances of yawn contagion.
"We had to sit or stay in presence of people and observe and take note (on the laptop, cell phone, notebook, depending on the situation)," Norscia explained in an email.
The researchers found that there was no difference in the rate of spontaneous yawning -- without a social trigger -- between men and women. But women responded much more frequently to another person's yawn (on average, 54% of the time) than men did (41%). Also, people were more likely to yawn in response to friends' yawns (48%) than they were to those of acquaintances (41%), and even more likely to respond to the yawns of romantic partners or family members (55%).
"Empathy enhances parental care, interindividual communication and group living, by motivating prosocial behaviors and favoring the development of moral reasoning," the authors wrote.
The same capacity that gives women their comparatively better ability to empathize, also evident in their susceptibility to social yawning, could have broader social applications, the authors wrote.
"The ability to preconsciously decode and replicate the emotions of others, e.g. via yawn contagion and facial mimicry, may allow women to respond with more appropriate behaviors toward others and to be more successful in forming enduring alliances," the authors wrote.
The next step, Norscia said, is to see if this pattern holds up in other settings, across ethnicities, and even across species.
"We will keep on collecting observational data in natural settings and we will also move to more controlled experimental conditions (e.g. using yawning videos) to see how yawn contagion varies across categories, for example ethnic groups," he wrote. "We also study monkeys, apes and lemurs and it will be interesting to shed light on the biological basis of both spontaneous yawning and contagion, when present."