Abstract
Both psychology and historical studies have addressed the question of the diffusion of collective action events, although using very different methodological approaches and with differing concepts. In the present paper, we present a novel approach, combining historiographical research methods with analytic concepts from social psychology, to explore the psychological processes underlying riot diffusion. Using archive data from the 1831 wave of 'reform' riots, thick description of two collective action events provides evidence that the purpose of participants' actions was to prevent troops from passing through their towns to put down riots elsewhere. Their actions to support rioters in another location involved risk to themselves, and so can't easily be explained in terms of personal or local self-interest. Instead, the evidence-in the form of context, utterances, and observations-is more consistent with the idea of common identity between people in the different locations motivating pre-emptive solidarity, inadvertently spreading the riots. The use of historical archive data and historiographical research methods, suggesting a previously undocumented form of solidarity between participants at riot events, contributes to new understandings of the diffusion of collective action and how to study it in both historical studies and social psychology.