Latitude, population size, and the language-farming dispersal hypothesis

Question: The language-farming dispersal hypothesis postulates that the current biogeographic distribution of global ethnolinguistic diversity is due to prehistoric demic expansions of agricultural populations. Does human population size increase as diet is increasingly based on agriculture, and is this relation moderated by agricultural potential, as predicted by the language-farming dispersal hypothesis? Do these patterns hold constant within and between language families? Data: We use a global sample of 805 subsistence-level human societies in 160 language families. Method: We regress population size on latitude, agricultural dependence, and their interaction. We then analyse variation within and between language families. Conclusions: Our results provide strong support for the language-farming dispersal hypothesis and suggest that while human population sizes increase rapidly with agriculture, much of this effect is driven by latitudinal variation in agricultural potential. The resulting patterns in the ethnographic and ethnolinguistic records reflect competition for space between populations with different subsistence strategies and varying competitive abilities.