A human behavior that's often contagious may provide a simple clue to whether a person is a psychopath, researchers from Baylor University have discovered, per Smithsonian. A study published in the Personality and Individual Differences journal finds that individuals with psychopathic traits—including coldheartedness, self-centered impulsivity, and "fearless dominance," per a press release—are less likely to "catch" a yawn from other people, an action that has suggested empathy and emotional connections with other people in past studies, Smithsonian explains. "I thought, 'If it's true that yawning is related to empathy, I'll bet that psychopaths yawn a lot less.' So I put it to the test," the study's lead author, Brian Rundle, says in the release.
How he tested his hypothesis: After administering a psychological test to 135 students to see how they placed on the psychopath spectrum, his team applied electrodes to the subjects and placed them in front of computer screens that showed short clips of faces that were yawning, laughing, and neutral. The electrodes measured muscle and nerve reactions, plus how frequently subjects yawned. Researchers found the less empathy subjects had demonstrated on the test, the less likely they yawned. That doesn't mean if you don't catch a yawn, you're a psychopath, Rundle says. "The take-home lesson is not that if you yawn and someone else doesn't, the other person is a psychopath," he says in the release. "A lot of people didn't yawn, and we know that we're not very likely to yawn in response to a stranger we don't have empathetic connections with. … This is [just] a good starting point to ask more questions." (Yawning also gets less contagious as we age.)