(This paper is a companion paper to: “Cross-Cultural Evidence of Cognitive Adaptations for Social Exchange among the Shiwiar of Ecuadorian Amazonia”, by L. Sugiyama, J. Tooby, & L. Cosmides)

Selective Impairment of Reasoning About Social Exchange in a Patient with Bilateral Limbic System Damage

 Valerie E. Stone*, Leda Cosmides§, John Tooby§, Neal Kroll#, &Robert T. Knight||

*Department of Psychology, University of Denver, §Center for Evolutionary Psychology, University of California, Santa Barbara, ||Department of Psychology, University of California, Berkeley, #Department of Psychology, University of California, Davis


Social exchange is a pervasive feature of human social life.  Models in evolutionary biology predict that for social exchange to evolve in a species, individuals must be able to detect cheaters (non-reciprocators).  Previous research suggests that humans have a cognitive mechanism specialized for detecting cheaters.  Here we provide neurological evidence indicating that social exchange reasoning can be selectively impaired while reasoning about other domains is left intact.  The patient, R.M., had extensive bilateral limbic system damage. We compared his performance on two types of reasoning problem that were closely matched in form and equally difficult for control subjects: social contract rules (of the form, “If you take the benefit, then you must satisfy the requirement”) and precaution rules (of the form, “If you engage in hazardous activity X, then you must take precaution Y.”)  R.M. performed significantly worse in social contract reasoning than in precaution reasoning, when compared both to normal controls and to other brain-damaged subjects.  This dissociation in reasoning performance provides evidence that reasoning about social exchange is a specialized and separable component of human social intelligence, and is consistent with other research indicating that the brain processes information about the social world differently from other types of information. 

The article can be downloaded from PNAS Online.  Otherwise, for reprint requests contact Dr. Valerie Stone, vstone@du.edu