From Nature Human Behavior:
The global ecology of differentiation between us and them
Evert Van de Vliert
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).
Not having access to the whole article, I’m uncertain whether this is saying that there is more ingroup differentiation nearer the equator (e.g., clannish Sicily vs. nationalist/universalist Sweden, fractious India vs. imperial China), which seems plausible.
Or perhaps it is saying that there is more geographic differentiation along a north-south axis than along an east-west one: e.g., Italy is a difficult place to rule if you aren’t the Romans because it has a lot of north-south divergence. Similarly, the former country of Sudan broke apart into a brown north and a black south. This also sounds plausible.
Ukraine, which is 800 miles from east to west, is one country that has east-west problems. Germany also has had east-west divergence, in part because rivers in Germany tend to run from south to north, so the Rhine, for instance, created a fairly cohesive western Germany.