Several theories have been proposed for why religions and religious beliefs have evolved, but before now none of them have involved parasites.
Previous theories have suggested that religions help enforce group cooperation. Another suggestion is that religious thoughts and practices are a side-effect of mental abilities that have evolved for other purposes. For example, prayer is a small step from our evolved ability to rehearse what we plan to say to someone who isn't physically with us right now.
Crucially, none of these accounts can readily explain why the diversity of religions varies so much around the world. Brazil, for example, has 159 religions compared with Canada's 15, even though both countries are of comparable size.
Now Corey Fincher and Randy Thornhill have 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.
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.
Corey L. Fincher, Randy Thornhill (2008). Assortative sociality, limited dispersal, infectious disease and the genesis of the global pattern of religion diversity Proceedings of the Royal Society B: Biological Sciences, 275 (1651), 2587-2594 DOI: 10.1098/rspb.2008.0688