Intelligence affects the social outcomes of groups. A systematic study of the link is provided in an experiment where two groups of subjects with different levels of intelligence, but otherwise similar, play a repeated prisoner’s dilemma. Initial cooperation rates are similar, but increase in the groups with higher intelligence to reach almost full cooperation, while they decline in the groups with lower intelligence. Cooperation of higher intelligence subjects is payoff sensitive and not automatic: in a treatment with lower continuation probability there is no difference between different intelligence groups.