According to a new study, psychopaths - people characterised by a lack of empathy and abnormal social behaviours - may be more immune to "contagious yawning" than the rest of us.
Psychologists from Baylor University in Texas showed that people with 'psychotic' tendencies were less likely to engage in the natural behaviour of "catching" yawns.
Contagious yawning is seen as a sign of empathy and a means of social bonding. Even some animals, including chimps and dogs, display this behaviour.
"Psychopaths are partly defined by a lack of empathy and compassionate understanding of the feelings of others," Brian Rundle, one of the study's authors, told the Huffington Post.
"The fact that they aren't able to contagious yawn suggests that the mechanics involved in the empathy process aren't functioning normally."
Yawning video clips
The study, published in the journal Personality and Individual Differences, asked 135 college students to complete a Psychopathic Personality Inventory which tests for certain psychopathic traits such as coldheartedness, Machiavellianism, egocentricity, callousness and impulsivity.
As hypothesised, the researchers found that those who had more psychopathic qualities were less likely to yawn when they viewed the yawning clip.
"We can't say that if you don't yawn, you're a psychopath," Rundle said. "But it does give neuroscientists a better understanding of the mechanisms involved in both psychopathy and yawning independently, as well as in relation to each other."