The making of strange bedfellows: spatial diffusion in unorthodox coalition building

Why do ideologically distant political adversaries sometimes choose to govern together? This article advances research on coalition formation in parliamentary democracies by introducing a diffusion-of-innovation perspective. We argue that cooperation between political archenemies can spread spatially as parties learn from nearby cases. When sister parties in adjacent jurisdictions successfully form such coalitions without electoral punishment or sanction from national leaderships, the normative barriers to similar cooperation are weakened. In this way, unorthodox coalitions become politically imaginable and strategically viable through processes of observation, emulation, and learning. We test this argument in the context of Swedish local government, where an extraordinary transformation has occurred over the past few decades. Historically, cooperation between the Social Democrats and the Moderates – the two dominant rivals for national power – has been virtually unthinkable. Today, however, these former adversaries co-govern in nearly one-fifth of Sweden’s 290 municipalities. This shift in coalition behaviour challenges core expectations in coalition theory, which predict alliances between ideologically proximate parties. Drawing on a unique panel dataset covering eight local election cycles (1994–2022), we employ complementary log-log regression models to test whether exposure to unorthodox coalitions in the surrounding regional environment increases the likelihood of similar coalition formation. The analysis reveals clear evidence of hierarchical regional contagion: Social Democrat–Moderate coalitions are significantly more likely to emerge in administrative regions where similar arrangements existed in the previous term. These findings suggest that coalition formation is a socially embedded process of spatial learning, offering a new account of how political cooperation evolves across space.