A University of Southern California study has found the first proof of structural brain abnormalities in people who habitually lie, cheat and manipulate others.
While previous research has shown that there is heightened activity in the prefrontal cortex - the area of the brain that enables most people to feel remorse or learn moral behavior - when normal people lie, this is the first study to provide evidence of structural differences in that area among pathological liars.
The research - led by Yaling Yang and Adrian Raine, both of the USC College of Letters, Arts and Sciences - is published in the October issue of the British Journal of Psychiatry.
The subjects were taken from a sample of 108 volunteers pulled from Los Angeles' temporary employment pool. A series of psychological tests and interviews placed 12 in the category of people who had a history of repeated lying (11 men, one woman); 16 who exhibited signs of antisocial personality disorder but not pathological lying (15 men, one woman); and 21 who were normal controls (15 men, six women).
"We looked for things like inconsistencies in their stories about occupation, education, crimes and family background," said Raine, a psychology professor at USC and co-author of the study.
"Pathological liars can't always tell truth from falsehood and contradict themselves in an interview. They are manipulative and they admit they prey on people. They are very brazen in terms of their manner, but very cool when talking about this."
Aside from having histories of conning others or using aliases, the habitual liars also admitted to malingering, or telling falsehoods to obtain sickness benefits, Raine said.
After they were categorized, the researchers used Magnetic Resonance Imaging to explore structural brain differences between the groups. The liars had significantly more "white matter" and slightly less "gray matter" than those they were measured against, Raine said.
Specifically, liars had a 25.7 percent increase in prefrontal white matter compared to the antisocial controls and a 22 percent increase compared to the normal controls. Liars had a 14.2 percent decrease in prefrontal gray matter compared to normal controls.
More white matter - the wiring in the brain - may provide liars with the tools necessary to master the complex art of deceit, Raine said.
"Lying takes a lot of effort," he said.
"It's almost mind reading. You have to be able to understand the mindset of the other person. You also have to suppress your emotions or regulate them because you don't want to appear nervous. There's quite a lot to do there. You've got to suppress the truth.
"Our argument is that the more networking there is in the prefrontal cortex, the more the person has an upper hand in lying. Their verbal skills are higher. They've almost got a natural advantage."
But in normal people, it's the gray matter - or the brain cells connected by the white matter - that helps keep the impulse to lie in check.
Pathological liars have a surplus of white matter, the study found, and a deficit of gray matter. That means they have more tools to lie coupled with fewer moral restraints than normal people, Raine said.
"They've got the equipment to lie, and they don't have the disinhibition that the rest of us have in telling the big whoppers," he said.
"When people make moral decisions, they are relying on the prefrontal cortex. When people ask normal people to make moral decisions, we see activation in the front of the brain," he explained. "If these liars have a 14 percent reduction in gray matter, that means that they are less likely to care about moral issues or are less likely to be able to process moral issues. Having more gray matter would keep a check on these activities."