Fincher & Thornhill (F&T) present a powerful case for the relationship between parasite-stress and religiosity. We argue, however, that the United States may be more religious than can be accounted for by parasite-stress. This greater religiosity might be attributable to greater sensitivity to immigration, which may hyperactivate evolved mechanisms that motivate avoidance of potential carriers of novel parasites.

Analysis of individual differences in religious observance in a Belizean community showed that the most religious (pastors and church workers) reported more illnesses, and that there was no tendency for the religiously observant to restrict their interactions to family or extended family. Instead, the most religiously observant tended to have community roles that widened their social contact: religion did not aid isolation – thus violating a key assumption of the parasite-stress theory of sociality.

In the target article, we presented the hypothesis that parasite-stress variation was a causal factor in the variation of in-group assortative sociality, cross-nationally and across the United States, which we indexed with variables that measured different aspects of the strength of family ties and religiosity. We presented evidence supportive of our hypothesis in the form of analyses that controlled for variation in freedom, wealth resources, and wealth inequality across nations and the states of the USA. Here, we respond to criticisms from commentators and attempt to clarify and expand the parasite-stress theory of sociality used to fuel our research presented in the target article.