Throughout the world people differ in the magnitude with which they value strong family ties or heightened religiosity. We propose that this cross-cultural variation is a result of a contingent psychological adaptation that facilitates in-group assortative sociality in the face of high levels of parasite-stress while devaluing in-group assortative sociality in areas with low levels of parasite-stress. This is because in-group assortative sociality is more important for the avoidance of infection from novel parasites and for the management of infection in regions with high levels of parasite-stress compared with regions of low infectious disease stress. We examined this hypothesis by testing the predictions that there would be a positive association between parasite-stress and strength of family ties or religiosity. We conducted this study by comparing among nations and among states in the United States of America. We found for both the international and the interstate analyses that in-group assortative sociality was positively associated with parasite-stress. This was true when controlling for potentially confounding factors such as human freedom and economic development. The findings support the parasite-stress theory of sociality, that is, the proposal that parasite-stress is central to the evolution of social life in humans and other animals.

Fincher & Thornhill's (F&T's) central hypothesis is that strong in-group norms were formed in part to foster parochial social alliances so as to enable cultural groups to adaptively respond to parasite stress. Applied to ancestral hominid environments, the story fits with evolutionary theory and the fragmentary data available on early hominid social formations and their geographical distributions. Applied to modern social formations, however, the arguments and inferences from data are problematic.

This commentary proposes experiments to examine connections between the presence of out-group members, neurovisceral reactions, religiosity, and ethnocentrism, to clarify the meaning of the correlational findings presented in the target article. It also suggests different ways of describing religious socialization and of viewing assertions about religion and health or about the human ability to detect pathogens.

Evolutionary socioecological theory and research proposing linking parasites with human social organization is uncommon and therefore welcome. However, more generally, condition-dependent adaptive phenotypic plasticity requires environmental uncertainty on a small scale, accompanied by reliable cues. In addition, genes in parasites may select among biologically adaptive cultural alternatives directly without necessarily going through human genetic predispositions, resulting in inter-specific gene-culture coevolution.

The target article gives two explanations for the correlation between pathogens, family ties, and religiosity: one highlights the benefits of xenophobic attitudes for reducing pathogen exposure, the other highlights the benefits of ethnic loyalty for mitigating the costs when a person falls ill. Preliminary data from traditional societies provide some support for the former explanation but not the latter.

Within the same pathogen-stress framework as proposed by Fincher & Thornhill (F&T), we argue further that pathogen stress promotes matrilocal rather than patrilocal family ties which, in turn, slow down the process of modernity; and that pathogen stress promotes social learning or copying, including the adoption of foreign religions.

Re-analysis of the data provided in the target article reveals a lack of evidence for a strong, universal relationship between parasite stress and the variables relating to sociality. Furthermore, even if associations between these variables do exist, the analyses presented here do not provide evidence for Fincher & Thornhill's (F&T's) proposed causal mechanism.

We question the plausibility of Fincher & Thornhill's (F&T's) argument that localised pathogen-host coevolution leads to out-groups having pathogens more damaging than those infecting one's own family or religious group.

In this commentary we suggest that Fincher & Thornhill's (F&T's) parasite-stress theory of social behaviors and attitudes can be extended to mating behaviors and preferences. We discuss evidence from prior correlational and experimental studies that support this claim. We also reanalyze data from two of those studies using F&T's new parasite stress measures.

Fincher & Thornhill (F&T) present a compelling argument that parasite stress underlies certain cultural practices promoting assortative sociality. However, we suggest that the theoretical framework proposed is limited in several ways, and that life history theory provides a more explanatory and inclusive framework, making more specific predictions about the trade-offs faced by organisms in the allocation of bioenergetic and material resources.

Fincher & Thornhill (F&T) present a model of in-group assortative sociality resulting from differing levels of parasite-stress in differing geographical locations in the United States and the world. Their model, while compelling, overlooks some important issues, such as mutualistic associations with parasites that are beneficial to humans and how some religious practices increase parasite risk.

Group assortative biases are stronger in regions where pathogen stress has been historically prevalent. Pushing the logic of this approach, extensions should include investigations of how cultural norms related to prosociality and relational striving may also covary with regional pathogen stress. Likewise, the pan-specific observation that diseased animals show decreased motor activity to facilitate recovery suggests that norms relevant to sickness behaviors may also vary as a function of regional parasite stress.

The hypothesis that high levels of religiosity are partly caused by high disease loads is in accord with studies showing that societal dysfunction promotes mass supernaturalism. However, some cultures suffering from high rates of disease and other socioeconomic dysfunction exhibit low levels of popular religiosity. At this point, it appears that religion is hard pressed to thrive in healthy societies, but poor conditions do not always make religion popular, either.

The link between parasite-stress and complex psychological dispositions implies that the social, political, and economic benefits likely to flow from public health interventions that reduce rates of non-zoonotic infectious disease are far greater than have traditionally been thought. We sketch a prudential and ethical argument for increasing public health resources globally and redistributing these to focus on the alleviation of parasite-stress in human populations.

At least four conceptually distinct mechanisms may mediate relations between parasite-stress and cultural outcomes: genetic evolution, developmental plasticity, neurocognitive flexibility, and cultural transmission. These mechanisms may operate independently or in conjunction with one another. Rigorous research on specific mediating mechanisms is required to more completely articulate implications of parasite stress on human psychology and human culture.

The evolution of religious traditions may be partially explained by out-group avoidance due to pathogen stress. However, many religious rituals may increase rather than decrease performers' susceptibility to infection. Moreover, religions often spread through proselytizing, which requires out-group interaction; and in other cases, the benefits of economic exchange increase religious pluralism and social interactions with out-groups.

Fincher & Thornhill's (F&T's) parasite-stress theory of sociality is supported largely by correlational evidence; its persuasiveness would increase significantly via lab and natural experiments and demonstrations of its mediating role. How the theory is linked to other approaches to group differences in psychological differences and to production and dissemination of cultural ideas and practices, need further clarification. So does the theory's view on the possible reduction of negative group interactions.

A 121-nation study of societal collectivism and a 174-nation study of political autocracy show that parasitic stress does not account for any variation in these components of culture once the interactive impacts of climatic demands and income resources have been accounted for. Climato-economic livability is a viable rival explanation for the reported effects of parasitic stress on culture.

Of the many far-reaching implications of Fincher & Thornhill's (F&T's) theory, we focus on the consequences of parasite stress for mating strategies, marriage, and the differing roles and restrictions for men and women. In particular, we explain how examination of cultures of honor can provide a theoretical bridge between effects of parasite stress and disproportionate emphasis on female purity.

Fincher & Thornhill's (F&T's) model is not entirely supported by common patterns of affect behaviors among people who live under varying climatic conditions and among people who endorse varying levels of (Western) religiosity and conservative political ideals. The authors' model is also unable to account for intra-regional heterogeneity in assortative sociality, which, we argue, can be better explained by a framework that emphasizes the differential expression of fundamental social cues for maintaining distinct social network structures.