Evidence is accumulating that, in the modern international system, democracies rarely fight each other. But the reasons for the phenomenon are not well understood. This article explores a similar phenomenon in other societies, using cross-cultural ethnographic evidence. It finds that polities organized according to more participatory (“democratic”) principles fight each other less often than do polities organized according to hierarchical principles. Stable participatory institutions seem to promote peaceful relations, especially if people perceive that others also have some control over politics.