The paper focuses attention on a poorly understood dynamic aspect of contentious politics: the interaction between the diffusion of new forms of protest behavior and the diffusion of police practices. It addresses this issue by giving specific attention to “double diffusion”: the interactions between innovative protester behavior and police responses. Different causal mechanisms of diffusions are mapped looking in particular at changes in protester/police interaction surrounding transnational countersummits organized around international events like the WTO Millenium Round in Seattle, G-8 and G-20 meetings, and conferences of the World Bank and the IMF, or the EU. We will identify three main mechanisms of protester diffusion -- promotion, adaptation and theorization and three main mechanisms of police response -- promotion and training, strategizing, and generalization The paper is structured as follows. After introducing our theoretical frame in Part One, begin in Part Two with two vignettes representing two turning points of the new protest repertoire in the United States and Europe – the Seattle and Genoa countersummits. We then turn in Part Three to a brief analysis of the evolving performances observed at countersummits, using Seattle and Genoa as our main data points. In Part Four, we turn to the mechanisms of diffusion we deduce from what followed. In Parts Five and Six, we examine the police practices that developed in response to these innovations and how these practices diffused. Finally, we suggest how interactive social learning occurred between police forces and transnational protesters during this period.
