The threat of disease could have driven the evolution of war - at least within a nation.This controversial idea is the brainchild of Randy Thornhill, an evolutionary biologist at the University of New Mexico in Albuquerque. He argues that cultures become more insular and xenophobic where diseases and parasites are common, preferring to drive away strangers who may carry new diseases. In contrast, cultures with a low risk of disease are more open to outsiders. Thornhill thinks these attitudes to outsiders colour each culture's propensity for war.Sure enough, when Thornhill and his colleagues gathered data from 125 civil wars, they found that such wars were far more common in nations with higher rates of infectious disease, such as Indonesia and Somalia.Participants at the conference at the University of Oregon in Eugene greeted Thornhill's theory with interested scepticism. It is "a very different way of thinking that has to be taken seriously", says primatologist Francis White who works at the university. John Orbell, a political scientist also at the university, says the idea is "pretty persuasive".Thornhill admits his ideas are hard to test, because countries with high disease levels are often poor, multi-ethnic and authoritarian, all of which can drive civil unrest. However, he says, when infectious disease fell in western nations in the 20th century thanks to antibiotics and sanitation, those same societies also became less xenophobic.