  Societies change through time, entailing changes in behaviors and
institutions. We ask how social change occurs when behaviors and institutions
are interdependent. We model a group-structured society in which the
transmission of individual behavior occurs in parallel with the selection of
group-level institutions. We consider a cooperative behavior that generates
collective benefits for groups but does not spread between individuals on its
own. Groups exhibit institutions that increase the diffusion of the behavior
within the group, but also incur a group cost. Groups adopt institutions in
proportion to their fitness. Finally, cooperative behavior may also spread
globally. As expected, we find that cooperation and institutions are mutually
reinforcing. But the model also generates behavioral source-sink dynamics when
cooperation generated in institutional groups spreads to non-institutional
groups, boosting their fitness. Consequently, the global diffusion of
cooperation creates a pattern of institutional free-riding that limits the
evolution of group-beneficial institutions. Our model suggests that, in a
group-structured society, large-scale change in behavior and institutions (i.e.
social change) can be best achieved when the two remain correlated, such as
through the spread successful pilot programs.
