Two hypotheses, not one

The authors cite three core features of in-group assortative sociality: philopatry (reduced mobility outside the natal group), xenophobia, and ethnocentrism. The rationale given for the association of pathogens with philopatry and xenophobia is the avoidance of exposure to novel strains of pathogens to which the individual has poor resistance. A different argument is given for ethnocentrism: Close relationships with other group members help people to survive infectious diseases. In the target article, pathogens are hypothesized to lead to greater religiosity through both mechanisms, whereas close family ties would seem to be most directly related to in-group loyalty.

These are very different arguments, with different implications. The former explanation (reducing exposure through reduced contact with outsiders) is highly specific to pathogens, especially non-zoonotic pathogens, which adds to its power. The latter explanation (the benefits of social support) is weaker, since social support would be valuable in coping with many sources of stress in addition to infectious disease. It is helpful to keep these two lines of argument separate because, as the authors note in their conclusion, they can lead to different predictions.

An anthropological perspective (and preliminary results)

Data supporting the relationship between pathogens and in-group assortative sociality have come almost exclusively from modern state-level societies, but if the behaviors are part of our evolved psychology they were presumably shaped in contexts closer to those found in traditional anthropological populations. In these societies, mortality from pathogens is a greater threat, and there are fewer non-social means of buffering risks of all sorts. The need for social mechanisms to cope with the threat of infectious disease, therefore, should be especially important in these societies.

In a recent preliminary analysis using the Standard Cross-Cultural Sample of traditional societies, Cashdan and Steele (Reference Cashdan and Steele2010) found support for the argument that individuals will reduce exposure to novel pathogens through reduced mobility and xenophobia. Mobility among communities (a more direct measure than range size) was inversely correlated with pathogen load, and the relationship held when controlling for latitude (no relationship was found with inter-group contact). There was also some evidence, albeit weaker, for a relationship between overall pathogen risk and xenophobia (also controlling for latitude). Both of these support the argument that behaviors and attitudes that reduce contact with members of other groups will be more prevalent where pathogen risk is greater.

Does pathogen risk also foster ethnocentrism? In this preliminary study (Cashdan & Steele Reference Cashdan and Steele2010), no relationship existed between pathogen stress and ethnocentrism (ethnic loyalty) at either the local or regional level. Since ethnocentrism and xenophobia were uncorrelated in the Standard Cross-Cultural Sample (Cashdan Reference Cashdan2001b), the data permit the isolation of the two lines of argument highlighted above. They suggest that the relationship between pathogens and in-group assortative sociality (including religiosity) is likely to be driven primarily by the way the latter reduces exposure to other groups, and that other related variables may be correlated with pathogens because they are correlated with traits that reduce this exposure.