This study developed and tested the idea that social categorization processes help to explain framing effects in social dilemmas. Using self-categorization theory, we predicted that members of a collective would cooperate more when they faced homogeneous rather than heterogeneous resource frames. We manipulated the starting point of resources for the members of each of two subgroups making up a larger collective in a nested social dilemma. The resources of the subject's own subgroup started at the individual, subgroup, or collective level, and the resources of the opposing subgroup also started at the individual, subgroup, or collective level. The condition in which both subgroups' resources began at the collective level was similar to a traditional commons dilemma, while the condition in which both subgroups' resources began at the individual level was similar to a traditional public goods problem. The other conditions were new to this research domain. We found higher allocations to the collective account when the initial resource location was the same for both subgroups (versus different for each subgroup), even when this initial location was the subgroup or individual level. Whereas previous explanations for framing effects cannot account for these results, social categorization processes provide a parsimonious explanation for the observed pattern of allocations. Copyright 1999 Academic Press.