Most historians have abandoned the search for general principles governing the evolution of human societies. A typical approach to studying why institutions (laws, rules, sanctions, customs, and norms) emerge, change, and disappear is to focus on explanations that are contingent on the specific historical circumstances in which such institutions evolve. However, although every society is unique in its own ways, this doesn’t preclude the possibility that common features are independently shared by multiple societies. In my presentation I will argue that it is possible to study both the diversity and commonalities in social arrangements found in the human past. To advance beyond purely theoretical debates and comparisons based on limited samples, my colleagues and I are building a massive repository of systematically collected, structured historical and archaeological data, Seshat: Global History Databank. Specifically, I will focus on the evolution of institutions that promote equality (or vice versa, inequality). Levels of inequality have changed dramatically during the past 10,000 years of human evolution: from egalitarian small-scale societies of hunter-gatherers to first hierarchical societies with great inequities in the distribution of power, status, and wealth. The Axial Age (c.800–200 BCE) introduced another notable transformation, starting a move towards greater egalitarianism that has been continuing to the present. I will describe how the Seshat project codes data on religion, norms and institutions, and other cultural characteristics of historical societies in a form that make them suitable for statistical analyses, and present preliminary results of testing different theories explaining the evolution of a particular equity institution with these data.