Enhanced recognition of defectors depends on their rarity.

Evolutionary psychologists have proposed that humans possess cognitive mechanisms for social exchange, but have perhaps focused overmuch on "cheating", because avoiding exploitation in reciprocal exchange could be accomplished either by avoidance of defectors or by attraction to cooperators. Past studies that have claimed to support the existence of a "cheater-detection module" by finding enhanced memory for the faces of "cheaters" have mostly relied on verbal descriptions, and these are prone to bias if the degree of cheating is unintentionally more severe than the degree of cooperation. Given that populations differ in the prevalence of defectors, it is most effective to remember whatever type is rare rather than always focus on cheaters. In the present experiment, participants played a computerized trust game and saw faces of cooperators and defectors in 20%/80%, 50%/50%, or 80%/20% ratios. Consistent with predictions, defectors were remembered best when rare but worst when common, supporting the existence of slightly more general cognitive mechanisms rather than specific cheater-recognition mechanisms.