Both social cooperation and self!control require reciprocation[ In social coop! eration situations a single person|s cooperative act\ if not reciprocated by others\ would be unreinforced both immediately and in the long term[ Similarly\ a single act of self!control "refusing a single cigarette\ for example#\ if not followed up by other such acts\ would also be unreinforced immediately and in the long term[ The present experiments varied probability of reciprocation in a prisoner|s dilemma! like game that could resemble a social cooperation or a self!control situation[ Participants earned points by playing against a computer[ The computer o}ered the participant a choice between 4 or 5 points "{cooperation| by the computer# or between 0 or 1 points "{defection| by the computer#[ The participant|s choice of the lesser alternative "{cooperation|# or the greater alternative "{defection|# on the current trial led\ respectively\ to cooperation or defection by the computer on the next trial with a probability "probability of reciprocation\ PR# which could vary[ When PR was greater than 52)\ consistent cooperation maximized the par! ticipant|s earnings[ When PR was less than 52)\ defection maximized earnings[ Three conditions were studied] PR signaled by spinners^PR unsignaled^PR unsig! naled with participants believing that they were playing with another person\ not the computer[ With PR 099) "{tit!for!tat|# and PR 49)\ the modal participant maximized earnings under all three conditions[ With PR 64)\ par! ticipants maximized earnings only when PR was signaled[ These results indicate that di}erences between people|s tendency to cooperate with other people "social cooperation# and their tendency to cooperate with their own future interests "self! control# may lie in di}erences in subjective PR [ Copyright ร 1990