I know this paper will make some ScienceBloggers very happy, Assortative sociality, limited dispersal, infectious disease and the genesis of the global pattern of religion diversity:
Why are religions far more numerous in the tropics compared with the temperate areas? We propose, as an answer, that more religions have emerged and are maintained in the tropics because, through localized coevolutionary races with hosts, infectious diseases select for three anticontagion behaviours: in-group assortative sociality; out-group avoidance; and limited dispersal. These behaviours, in turn, create intergroup boundaries that effectively fractionate, isolate and diversify an original culture leading to the genesis of two or more groups from one. Religion is one aspect of a group's culture that undergoes this process. If this argument is correct then, across the globe, religion diversity should correlate positively with infectious disease diversity, reflecting an evolutionary history of antagonistic coevolution between parasites and hosts and subsequent religion genesis. We present evidence that supports this model: for a global sample of traditional societies, societal range size is reduced in areas with more pathogens compared with areas with few pathogens, and in contemporary countries religion diversity is positively related to two measures of parasite stress.
There are more religions in the tropics, but I'm not sure they have the causality right. For example, pathogens could lead to lower IQ, which may lead to more religion (there are 1,000 other correlations one could make like this). But with Toxoplasma gondii as a prior I'm not willing to dismiss the sort of argument as the one made above on first inspection.
Does religious schismaticity (note: I will not defend that word, but can't think of a better) really constitute "anticontagion behavior"?
You can catch a bug from somebody by fighting with him at least as easily as by sharing a beer.
This seems to me a very weird paper. The argument involves a remarkable number of un-compelling assumptions. I find it hard to imagine how they came up with the hypothesis, or the way they have tested it. Probably, maybe, it contains some concealed post hoc reasoning (constructing an hypothesis to make your results look more interesting). But maybe it is a brilliant piece of insight which succeeds in jumping several steps at once. Time will tell. But its good that Proceedings B will still publish such speculative work (perhaps it also helps to have a famous author).
For a really good illustration of religion splitting, check out the video posted at Pharyngula.