Waves of collective mobilization, when participation increases rapidly and expectations shift dramatically, pose an important puzzle for social science. Such waves, I argue, can only be explained by an endogenous process of 'positive feedback.' This article identifies two distinct mechanisms—interdependence and inspiration—which generate positive feedback in collective mobilization. It also provides a detailed analysis of one episode: the wave of strikes that swept American cities in May 1886. Although historians and sociologists have suggested various precipitants, these do not account for the magnitude of the upsurge. Focusing on events in Chicago during the months before May, the article provides quantitative and qualitative evidence for positive feedback.

Michael Biggs, Department of Sociology, University of Oxford