From The Economist:
"SOME people, notably Richard Dawkins, an evolutionary biologist at Oxford University, regard religion as a disease. It spreads, they suggest, like a virus, except that the “viruses” are similar to those infecting computers—bits of cultural software that take over the hardware of the brain and make it do irrational things.
Corey Fincher, of the University of New Mexico, has a different hypothesis for the origin of religious diversity. He thinks not that religions are like disease but that they are responses to disease—or, rather, to the threat of disease. If he is right, then people who believe that their religion protects them from harm may be correct, although the protection is of a different sort from the supernatural one they perceive."
(I am skipping down to the evidence for this idea):
"The two researchers also looked at anthropological data on how much people in “traditional” (ie, non-urban) societies move around in different parts of the world. They found that in more religiously diverse (and more disease-ridden) places people move shorter distances than in healthier, religiously monotonous societies. The implication is that religious diversity causes people to keep themselves to themselves, and thus makes it harder for them to catch germs from infidels.
Of course, correlation is not causation. But religion is not the only cultural phenomenon that stops groups of people from mixing. Language has the same effect, and in another, as yet unpublished study Mr Fincher and Dr Thornhill found a similar relationship there too. Moreover, their search of the literature turned up work which suggests that xenophobia is linked psychologically with fear of disease (the dirty foreigner…). Perhaps, then, the underlying reason why there is so much hostility between ethnic groups is nothing to do with the groups themselves, but instead with the diseases they may bring."
As the last paragraph states, correlation is not causation. Perhaps xenophobia, be it racial, cultural, or otherwise, served our ancestors well**. It is well known that Europeans brought some nasty diseases to the Native Americans back in the day. Then again, perhaps this study is flawed; After all, the Journal of Religion and Society found that, "higher rates of belief in and worship of a creator correlate with higher rates of homicide, juvenile and early adult mortality, STD infection rates, teen pregnancy, and abortion in the prosperous democracies."
** I want to make it known that I do not approve of racism or any other prejudice even if it had or has a biological benefit.