Demographic shifts, inter-group contact and environmental conditions drive language extinction and diversification

Humans collectively use thousands of languages. The number of languages in a region (i.e. 'richness') varies widely. Empirical research has identified social, environmental, geographic and demographic factors associated with language richness. However, our understanding of causal mechanisms and variation in their effects over space has been limited by prior analyses focusing on correlation and assuming stationarity. Here we use process-based, spatially explicit stochastic models to simulate the emergence, expansion, contraction, fragmentation and extinction of language ranges. We varied parameter settings in these computer-simulated experiments to evaluate the extent to which different processes reproduce observed patterns of language richness in North America. We find that the majority of spatial variation in language richness is explained by models in which environmental and social constraints determine population density, random shocks alter population sizes more frequently at higher population densities, and population shocks are more frequently negative than positive. Language diversification occurs when populations split after reaching size limits, and when ranges fragment due to population contractions following negative shocks or due to contact with other groups expanding following positive shocks. These findings support theories arguing that environmental and social conditions, constraints on group sizes, outcomes of contact and shifting demographics all shape language richness.