Movements develop in coevolution with regimes and other actors in their environments. Movement trajectories evolve through stochastic processes and are constrained, but not determined, by structures. Coevolution provides a theoretical structure for organizing existing understandings of social movements and sharpening future research. Stochastic thinking is essential for recognizing the both the volatility and path dependence of collective action and its underlying structural constraints. Formal models of diffusion, adaptive learning, mutual reinforcement, and inter-actor competition are developed and compared with empirical protest series. Responses to exogenous reinforcement, mutual adaptation in which failure is as important as success, and inter-actor competition are the most plausible mechanisms to account for empirical patterns. Trajectories of action depend upon the number of discrete random actors. Overall, the analysis suggests that movement dynamics are shaped more by interactions with other actors than by processes internal to a movement, and that empirical analysis must be sensitive to the level of aggregation of the data.

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