Humans possess a specialized brain module for reasoning in social situations, particularly for detecting individuals who cheat in social exchanges, according to the authors of two articles to be released the week of August 12-16, 2002 in the Proceedings of the National Academy of the Sciences (PNAS).
Valerie Stone, assistant professor of cognitive neuroscience at the University of Denver, and evolutionary psychologists Leda Cosmides and John Tooby, co-authors of both articles, have proposed that humans and other social primates have evolved specialized brain systems for processing social information, particularly a mechanism to detect when individuals violate the terms of social contracts. Now the team has produced evidence in support of its theory.
Stone and her colleagues present a case study of a patient (R.M.) with damage to the limbic system, a brain region critical for processing emotional and social information. R.M. performed normally on tasks that required him to determine whether an individual might be breaking a precautionary rule (example: "if you work with toxic chemicals then you have to wear safety gloves,"). But R.M. performed poorly on logically identical tasks that required him to determine whether an individual might be cheating on a social contract (example: "if you borrow my car then you have to fill the tank with gasoline.").
R.M.'s pattern of impaired reasoning could not occur if cheater detection depended solely on general reasoning abilities. Instead, his selective deficiency suggests that detecting social cheaters requires different neural circuitry than general reasoning.
An overlapping team of researchers, including Larry Sugiyama of the University of Oregon's Anthropology Department, then tested another prediction of the theory: that an enhanced ability to detect cheaters is found in all human cultures. They tested this among the Shiwiar, non-literate hunter-horticulturalists who live in a remote region of the Ecuadorian Amazon. A modified version of the task given to R.M. was used to measure their ability to detect social cheaters, and the results were compared to a previous study of Harvard undergraduates.
Although the Shiwiar were more likely than Harvard students to seek out information not relevant to the task, their ability to identify cheaters was equivalent.
Together, the studies show that the brain may use different systems for processing social information than non-social information. Reasoning about social exchange can be impaired without negatively affecting other reasoning abilities and cognitive specialization for such social rules occurs even in disparate cultural contexts.
The authors note that the ability to detect cheaters is a critical foundation for many forms of human cooperation.
The two studies to be released by PNAS include: "Selective impairment of reasoning about social exchange in a patient with bilateral limbic system damage," by Valerie E. Stone, Leda Cosmides, John Tooby, Neal Kroll and Robert T. Knight, and "Cross-cultural evidence of cognitive adaptations for social exchange among the Shiwiar of Ecuadorian Amazonia," by Lawrence S. Sugiyama, John Tooby and Leda Cosmides.
Stone's research will appear online and in the printed version of PNAS the week of August 12-16.
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