I argue that an ecologically distributed conception of instrumental rationality can and should be extended to a socially distributed conception of instrumental rationality in social environments. The argument proceeds by showing that the assumption of exogenously fixed units of activity cannot be justified; different units of activity are possible and some are better means to independently given ends than others, in various circumstances. An important social heuristic, the mirror heuristic, enables the flexible formation of units of activity in game theoretic (...) situations, including collective units where these are instrumentally effective. In effect, the mirror heuristic makes the formation of units of activity endogenous to instrumental rationality. Moreover, the mirror heuristic is a conditional metaheuristic, which depends on mind reading of the heuristics of other players rather than on predictions of their behavior. Such mind reading can be regarded as emerging from an arms race between behavioral mimicry and ever smarter behavior reading. Even though unilateral mind reading may have benefits, the mirror metaheuristic illustrates that mutual mind reading has distinctive functions in responding to the challenges of social complexity. If simple heuristics can make us smart in the right environments, then social heuristics can make us smarter still. (shrink)