The target article by Fincher & Thornhill (F&T) describes a parasite-stress theory of sociality stating that social life in humans (and other animals) is shaped by the demands of ecological adversity and infectious-disease stress. A discussion of the variables forming in-group assortative sociality in terms of parasite-stress – philopatry, ethnocentrism, and xenophobia – is at the center of the article. Through the use of large-scale data sets, the article provides compelling evidence from around the world of the relationship between parasite-stress and strength of family ties and religiosity. It proposes an account of human history (e.g. why people migrated from Africa to Eurasia) that is an alternative to other accounts such as those suggested by Diamond (Reference Diamond1998) and Inglehart (e.g., Inglehart & Baker Reference Inglehart and Baker2000).

F&T's ambitious theoretical account is tested primarily by the use of existing data sets on the hypothesized relationship between parasite-stress and in-group assortative sociality. This approach has both advantages and disadvantages. The advantages include large sample sizes, access to data collected in various parts of the world, thereby permitting comparisons between geographic locations (e.g., Africa, where parasite-stress is exceptionally high, versus other world regions, where it is not), and the ability to generalize the findings across many cases. Disadvantages include a reliance on the operational definitions of social variables of interest adopted by existing surveys, making it difficult to take into account cultural differences in survey responses and question comprehension, and the exclusion of experimental research.

The heavy reliance on surveys restricts the article to drawing on correlational empirical evidence. Exceptions are studies by Schaller et al. (Reference Schaller, Miller, Gervais, Yager and Chen2010) and Mortensen et al. (Reference Mortensen, Becker, Ackerman, Neuberg and Kenrick2010) which note a change in immediate immune response and behavioral actions following visual cues pertinent to the risk of parasitic infection. Given the nature of the data and the hypotheses tested (e.g., the impossibility to induce infectious diseases to examine resulting psychological consequences!), one might claim that correlational designs are most appropriate. However, F&T at times claim that the evidence is causal and should be interpreted as such. More experimental work is needed to be able to refer to the role of parasite-stress as a causal factor in certain aspects of human social life. The need of more causal evidence in this area of work should not be seen as limited to lab experiments. As the authors rightly note, some very persuasive causal evidence can come from natural experiments where changes in social variables of interest can be assessed after infectious disease levels are reduced or increased locally. Large-scale data sets may also provide a way into examining causality if assessments are repeated over time, allowing for across-time comparisons and taking into account possible changes in parasite-stress in certain regions. It would also be exciting, as the authors state, to find evidence of the explanatory value of parasite-stress as a mediating factor (e.g., between religiosity and health).

Further testing of the hypotheses put forward by the parasite-stress theory could occur in the area of migration. Do those who leave their in-group and venture into the unknown have lower perceived susceptibility to contracting infectious diseases? How do economic migrants compare to migrants whose mobility decisions are shaped by political or environmental reasons beyond their control? Do changes in social variables, such as religiosity and in-group family ties, occur exclusively as a result of mobility driven by emancipation from parasites?

One important claim of parasite-stress theory is that social variables such as xenophobia and ethnocentrism that can have devastating consequences for humanity (e.g., in the case of wars and genocides), are stronger in areas of high-parasite stress and thus have a protective function for in-group members. It would be useful to discuss whether there are any moderators in this straightforward parasite-stress and in-group assortative sociality link. What are the conditions under which the link gets stronger or weaker? Given the destructive nature of social variables such as xenophobia, what does the theory have to say about potential ways of reducing the negative relationships between in-groups and out-groups? Is this exclusively possible via reduction of levels of parasite-stress, or are there other ways to reduce the negative effects of in-group assortative sociality?

The search for the roots of differences in psychological processes between members of different cultural groups has been triggered by the accumulation of rich empirical evidence demonstrating substantial cross-cultural variation in such processes. I applaud F&T's important contribution to this search. In cultural psychology, this search has taken place primarily in a socio-cultural/ecological context. For example, among others, Kitayama et al. (e.g., Reference Kitayama, Mesquita and Karasawa2006; Reference Kitayama, Park, Sevincer, Karasawa and Uskul2009) have wondered about the roots of rugged individualism in the United States and suggested the frontier spirit as a possible explanation. Uskul et al. (Reference Uskul, Kitayama and Nisbett2008) have suggested that different levels of economic interdependence due to subsistence patterns might underlie cognitive differences typically observed in individualistic versus collectivistic communities (also see Berry Reference Berry1966; Berry et al. Reference Berry, van de Koppel, Sénéchal, Annis, Bahuchet, Cavalli Sforza and Witkin1986). Oishi (Reference Oishi2010) has demonstrated that residential mobility might be partly responsible for some defining characteristics of individualistic cultures. I invite F&T to look for linkages between socio-cultural/ecological and biological approaches to cross-cultural variations in psychological processes, which would be very fruitful for both cultural and evolutionary psychology and other related disciplines. A more detailed account of how the parasite-stress theory is linked with some of the major theoretical efforts that have been devoted to the understanding of the production and dissemination of cultural ideas and practices (e.g., Kitayama et al. Reference Kitayama, Conway, Pietromonaco, Park and Plaut2010, Richerson & Boyd Reference Richerson and Boyd2005, Schaller & Crandall Reference Schaller and Crandall2004, Sperber Reference Sperber1996) would also be welcome.