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.

Mr Fincher is not arguing that disease-protection is religion's main function. Biologists have different hypotheses for that. Not all follow Dr Dawkins in thinking it pathological. Some see it either as a way of promoting group solidarity in a hostile world, or as an accidental consequence of the predisposition to such solidarity. This solidarity-promotion is one of Mr Fincher's starting points. The other is that bacteria, viruses and other parasites are powerful drivers of evolution. Many biologists think that sex, for example, is a response to parasitism. The continual mixing of genes that it promotes means that at least some offspring of any pair of parents are likely to be immune to a given disease.

Mr Fincher and his colleague Randy Thornhill wondered if disease might be driving important aspects of human social behaviour, too. Their hypothesis is that in places where disease is rampant, it behoves groups not to mix with one another more than is strictly necessary, in order to reduce the risk of contagion. They therefore predict that patterns of behaviour which promote group exclusivity will be stronger in disease-ridden areas. Since religious differences are certainly in that category, they specifically predict that the number of different religions in a place will vary with the disease load. Which is, as they report in the Proceedings of the Royal Society, the case.

Proving the point involved collating a lot of previous research. Even defining what constitutes a religion is fraught with difficulty. But using accepted definitions of uniqueness, exclusivity, autonomy and superiority to other religions they calculated that the average number of religions per country is 31. The range, though, is enormous—from 3 to 643. Côte d'Ivoire, for example, has 76 while Norway has 13, and Brazil has 159 while Canada has 15. They then did the same thing for the number of parasitic diseases found in each country. The average here was 200, with a range from 178 to 248.

Obviously, some of the differences between countries are caused by differences in their areas and populations. But these can be accounted for statistically. When they have been, the correlation between the number of religions in a place and how disease-ridden it is looks impressive. There is less than one chance in 10,000 that it has come about accidentally.

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.