Infants use information about efficiency to identify agents’ physical goals. But how do they recognize actions with social rather than physical functions? They may rely on an understanding that socially meaningful actions work not by efficiently enacting physical changes, but instead through shared use across group members. We found support for this hypothesis across several experiments that probed the conditions under which 8- and 9-month-old infants expect an action to be performed by additional members of the initial actor's social group. Infants generalized actions to new members of social groups if and only if the actions in question werenon-instrumental and infants had observed two socially related individuals repeating the action, whether or not they were members of the group across which generalization was tested. Thus, infants use characteristics of social behavior – physical inefficiency and shared use by group members – to categorize actions as social.