Humans appear to have developed a brain mechanism to detect social cheating--a universal alarm bell that sounds when, for example, somebody slips off the wedding ring before entering a bar or crashes a potluck dinner without a covered dish. So find two recent studies co-authored by evolutionary psychologists Leda Cosmides, PhD, and John Tooby, PhD, of the University of California, Santa Barbara.

The studies, which appear in the Proceedings of the National Academy of Sciences (PNAS, Vol. 99, No. 17), uncover neuropsychological and anthropological evidence that people use different brain circuitry for social reasoning than they do for other types of reasoning.

The first paper, co-authored with Valerie Stone, PhD, a University of Denver assistant professor of cognitive science, presents a case study of "R.M.," a man with extensive damage to the limbic system. R.M. had injured this region of the brain--known for its social and emotional processing--in a serious bicycle accident.

The research team compared R.M.'s reasoning about social-exchange rules and safety rules. He had to determine, for example, whether people had violated such social rules as "if you borrow my car, then you should return it full of gas," and such safety rules as "if you work with toxic chemicals, then you should wear a safety mask."

R.M. performed normally on the safety tasks, as compared with 37 people without neuropsychological damage, and two others with different types of brain damage. But he performed significantly below average on the social-exchange tasks. To Cosmides, this suggests that R.M. lacks the "mind's reasoning mechanism for detecting when people violate social contracts." Providing further evidence that there is an evolved brain mechanism for detecting cheaters, she says, is the other PNAS study. That research explores the social versus straight-logic reasoning of the Shiwiar people, a nonliterate group living in remotest Equador.

In the study, headed by University of Oregon anthropologist Larry Sugiyama, PhD, and co-authored by Cosmides and Tooby, 21 Shiwiar adults considered such pictorially presented social contracts as "if you bring me a basket of fish, then you can borrow my motor boat." If, for example, the motorboat owner reneged on his promise to the fisherman--after he'd eaten the basket of fish--the Shiwiar quickly considered that cheating.

"They were just as good as detecting cheating as highly educated students in the developed world," says Cosmides. And that, to her, suggests that "people have developed an evolutionary strategy for determining when somebody has violated a tit-for-tat agreement. It appears to be buffered against cultural variation."

She believes, however, that humans aren't the only animals who practice social exchange and detect social cheating. Other primates appear to do it too, she notes. But, as with language, "in humans it is richly elaborated."