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.