It turns out that folks kinda hate their genetic neighbours.
Enrico Spolaore was one of the keynote speakers at the Australasian Public Choice Society Meetings in Melbourne last week. I'd not before seen his work on genetic distance, but it's rather interesting.
Genetic distance measures the number of generations back you have to go before two populations share common ancestors. So if two populations diverged only a very short time ago, like the Danes and the English, their measured genetic distance is short; if they diverged a very very long time ago, like the Australian aborigines and the Mbuti Pygmies of Africa, their measured distance is long. While this is related to geographical distance, it's far from perfectly correlated: Canada's Inuit are closer to Tibetans than they are to any of the other Amerindians; the English are closer to the northern Indians than they are to the Lapps (Finish); the Mongols are closer to the Japanese than they are to the Chinese; the Indians of south east India are closer to the Italians than they are to the Thai people or the South Chinese.
Spoloare and Wacziarg find that this measure of genetic distance predicts whether two populations will go to war, after controlling for the usual set of determinants of conflict like geographical proximity, shared borders, income differences, religion, language, and so on. All else equal, the more two populations are genetically proximate, the more likely they are to go to war and the less likely they are to vote together at the UN. If current patterns of war have affected measured genetic distance, then causality may be wrong, but they use genetic distance as of year 1500 as an instrument.
I'd worried that results might be drawn from a few places that have been strategically important going back well before 1500. For example, if the bridge from Africa to Asia Minor via Sinai and Israel has been strategically important for thousands of years and if similar populations have lived around there for a long time, then the correlation of genetic distance and conflict could have things the wrong way round: frequent conflicts in strategic regions bring genetic mixing, and if the regions' strategic importance continues from well prior to 1500 to present, then it wouldn't be that folks want to fight with their genetic neighbours, but folks who fight a lot become genetic neighbours. Controlling just for having a common border or just for geographical distance wouldn't quite cut it. But they find that the effect also holds for country pairs that do not share a border.
Why might we fight more with our nearer than our more distant cousins? Spolaore suggests that genetic closeness makes it more likely that we'd be in conflict over rivalrous resources. I wonder whether we couldn't imagine a pleistocene explanation: if there's a fertility advantage to outbreeding but not too far (the sweetspot between inbreeding depression and outbreeding depression), then our ancestors 90,000 generations back on the Savannah who raided more closely related neighbours would have had a slight advantage over those who raided groups too genetically distant. Run the mechanism for 90,000 generations, and you've a population that's keyed to want to raid folks who are more like them.
In the May QJE, Spolaore and Wacziarg found that genetic distance from the United States explains cross country income differences after correcting for geographical distance, climate, transportation costs, and measures of social distance (historical, religious, linguistic). Again, they argue that genetic distance may be the best measure of "slowly changing genealogically transmitted characteristics, including habits and customs" - the bits of culture we can't adequately otherwise measure. That's certainly possible, and cuts against my evolutionary biology explanation above, mostly because it's hard to come up with an ev bio explanation of why genetic distance from the US would correlate with income differences. The best explanation I'd have would be that it's proxying for differences in average IQ: also somewhat genetic, but at some of the more depressed ends of the scale almost certainly highly environmentally influenced). But that would be a bit of a wash: there are genetically distant places above the US (Hong Kong) and far below the US (Equatorial Guinea) in reported average national IQ. Spolaore's culture explanation seems the more plausible.
Spolaore gave one of the best plenary addresses I've ever seen. If you get a chance to see him give a talk, go.