Think you might be dating a psychopath? There are several telltale signs, including a tendency to lie, a charming but callous demeanor and an inflated ego.
Here's another one to watch out for: When you yawn, he or she doesn't yawn back.
New research finds that psychopaths -- people characterized by a lack of empathy and abnormal social behaviors -- may be more immune to "contagious yawning" than the rest of us.
In the study, which will be published in the journal Personality and Individual Differences in November, psychologists from Baylor University in Texas showed that people who are more coldhearted are less likely to engage in the natural behavior of "catching" yawns.
Most people find it difficult to resist yawning when the person they are with yawns or shows signs of being tired. Why? Contagious yawning is a sign of empathy and a means of social bonding. Even some animals, including chimps and dogs, display this behavior.
"Psychopaths are partly defined by lack of empathy and compassionate understanding of the feelings of others," Brian Rundle, a doctoral researcher at the university and one of the study's authors, told The Huffington Post in an email. "The fact that they aren't able to contagious yawn suggests that the mechanics involved in the empathy process aren't functioning normally."
But what about people who lack empathy? To find out, the researchers asked 135 students to fill out a questionnaire called the Psychopathic Personality Inventory, which is a set of questions used to diagnose psychopathy by identifying personality traits such as coldheartedness, Machiavellianism, egocentricity, callousness and impulsivity. Then, the students viewed videos of people making different facial movements, including yawning.
As hypothesized, the researchers found that those who had more psychopathic qualities were less likely to yawn when they viewed the yawning clip.
Of course, there's no reason to jump to conclusions. The fact that someone doesn't seem to be affected by contagious yawning doesn't necessarily make them a psychopath.
"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."