Of all the theories that have been put forward to explain how religions spread, I think this one, as reported in the British Psychological Society’s research blog, may be the most…sickening. In a nutshell, the article discusses a recent study that “tested the idea that religious diversity is a side-effect of the fragmentation of cultures that tends to occur in the face of increased threat from infectious disease.” In other words, there’s evidence to suggest that the variety of infectious parasites in a given region directly influences the variety of religions in that region. From the article:
Fincher and Thornhill used the World Christian Encyclopedia and the Global Infectious Disease and Epidemiology Network to compare the spread of infections and religions across 219 countries. Their results were clear: in regions with a greater variety of infectious parasites, the diversity of religions also tends to be greater. This association held strong even after exploring the impact of other potential factors, such as differences in democratisation and histories of colonisation.
The researchers say the association between religion and parasites occurs because reducing contact with outsiders can help protect against disease. In turn, when cultures fragment and groups avoid making contact with each other, more religions are likely to spring up.
“Although religion apparently is for establishing a social marker of group alliance and allegiance, at the most fundamental level, it may be for the avoidance and management of infectious disease,” Fincher and Thornhill said. The pair also believe that the diversity of languages and parasites tends to co-vary across the globe for similar reasons.
Bearing in mind that correlation is not causation, this is still pretty interesting. (Note, none of this is to be confused with the theory of religion as virus or virus of the mind, which are quite different, but also intriguing.)
In the same general genre, this Jared Diamond lecture on the evolution of religion is worthwhile.