The global ecology of differentiation between us and them.

Humans distinguish between we-groups and they-groups, such as relatives versus strangers and higher-ups versus lower-downs, thereby creating crucial preconditions for favouring their own groups while discriminating against others. Reported here is the finding that the extent of differentiation between us and them varies along latitude rather than longitude. In geographically isolated preindustrial societies, intergroup differentiation already peaked at the equator and tapered off towards the poles, while being negligibly related to longitude (observation study 1). Contemporary societies have evolved even stronger latitudinal gradients of intergroup differentiation (survey study 2 around 1970) and discrimination (mixed-method study 3 around 2010). The geography of contemporary differentiation and discrimination can be partially predicted by tropical climate stress (warm winters, hot summers and irregular rainfall), largely mediated by the interplay of pathogen stress and agricultural subsistence (explanatory study 4). The findings accumulate into an index of intergroup discrimination by inhabitants of 222 countries (integrative study 5).