Christianity spread faster in small, politically structured societies.

Over the past 2,000 years, Christianity has grown from a tiny Judaic sect to the world's largest religious family 1 . Historians and social scientists have long debated whether Christianity spread through a top-down process driven by political leaders or a bottom-up process that empowered social underclasses 2-6 . The Christianization of Austronesian populations is well-documented across societies with a diverse range of social and demographic conditions 7 . Here, we use this context to test whether political hierarchy, social inequality and population size predict the length of conversion time across 70 Austronesian cultures. We also account for the historical isolation of cultures and the year of missionary arrival, and use a phylogenetic generalized least squares method to estimate the effects of common ancestry and geographic proximity of cultures 8 . We find that conversion to Christianity typically took less than 30 years, and societies with political leadership and smaller populations were fastest to convert. In contrast, social inequality did not reliably affect conversion times, indicating that Christianity's success in the Pacific is not due to its egalitarian doctrine empowering social underclasses. The importance of population size and structure in our study suggests that the rapid spread of Christianity can be explained by general dynamics of cultural transmission.