Elsa Ermer and Kent Kiehl of the University of New Mexico say less than 1 percent of people are psychopaths -- who lack moral emotions, are impulsive, and routinely violate social and legal norms -- but they make up about 20 percent of the prison population.
Most people aren't very good at logical reasoning -- thinking through the implications of any rule in the form "if P, then Q" -- but people do better on logic problems that involve a social contract such as "If you borrow the car, then you have to fill up the tank with gas."
Ermer and Kiehl gave 67 prisoners -- some who were psychopaths and some who were not -- a series of problems involving descriptive rules, social contracts and precautions.
The study, published in the journal Psychological Science, found prisoners who weren't psychopaths did about as well as non-prisoners on all kinds of reasoning. Psychopaths scored similarly on straight logical reasoning, but did worse on social contracts and precautions.
"This work suggests that psychopaths don't understand cheating in the normal way, so they might not realize when they're cheating other people or when other people would react badly to cheating," Ermer says in a statement.
The inability of psychopaths to reason about precautions may help explain why they take risks and do impulsive things that get them in trouble, Ermer says.