Gods and Bugs
Corey Fincher and Randy Thornhill hypothesized 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 predicted that patterns of behaviour which promote group exclusivity are stronger in disease-ridden areas. Since religious differences are in that category, they predicted that the number of different religions in a place will vary with the disease load. [See "Assortative sociality, limited dispersal, infectious disease and the genesis of the global pattern of religious diversity."]
Using accepted definitions, 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 found that the average number of parasitic diseases per country is 200, with a range from 178 to 248.
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 number of religions in a place is strongly correlated with how disease-ridden it is. There is less than one chance in 10,000 that it has come about accidentally.
The 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.
“Praying for health,” The Economist