They invoke the optimal outbreeding hypothesis as to why out-group contact would be costly, but there is very scant evidence for this in humans. Then again, there's the paper I just posted about a few days ago that showed higher rates of obesity among multi-racial individuals, but I don't think that really counts in this context... Razib touches on some other potential examples here, ...as well on this paper.
Assortative sociality, limited dispersal, infectious disease and the genesis of the global pattern of religion diversity
Proceedings of the Royal Society, B First Cite
Abstract: 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.