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Abstract
Cooperation based on reciprocal altruism has evolved in only a small number of species,
yet it constitutes the core behavioral principle of human social life. The iterated
Prisoner's Dilemma Game has been used to model this form of cooperation. We used fMRI
to scan 36 women as they played an iterated Prisoner's Dilemma Game with another woman
to investigate the neurobiological basis of cooperative social behavior. Mutual cooperation
was associated with consistent activation in brain areas that have been linked with
reward processing: nucleus accumbens, the caudate nucleus, ventromedial frontal/orbitofrontal
cortex, and rostral anterior cingulate cortex. We propose that activation of this
neural network positively reinforces reciprocal altruism, thereby motivating subjects
to resist the temptation to selfishly accept but not reciprocate favors.