Fincher & Thornhill (F&T) present correlational cross-national data as evidence that parasite stress underlies certain cultural practices, such as religiosity and family coherence. They argue that the ancestrally evolved adaptive feelings of philopatry, ethnocentrism, and xenophobia limit the introduction of novel parasites to local populations.

Ethnocentrism was originally defined as “a view of things in which a group other than one's own is the center of everything, and all others, including one's own group, are scaled and rated with reference to it” (Sumner Reference Sumner1906, p. 13). Subsequently, this construct has been subdivided into positive ethnocentrism, denoting in-group attraction, affiliation, or “love,” and negative ethnocentrism, denoting out-group repulsion, aversion, or “hate” (Figueredo et al. Reference Figueredo, Andrezejczak, Jones and Smith-Castro2011a). In spite of this distinction, F&T conflate philopatry, ethnocentrism, and xenophobia as common “elements of in-group assortative sociality” (sect. 2.1, para. 6). Nevertheless, in a survey done of 30 ethnic groups in East Africa, Brewer and Campbell (Reference Brewer and Campbell1976) found that positive in-group biases were overall completely unrelated to social distance toward out-groups. Allport (Reference Allport1954) and Brewer (Reference Brewer1999) had previously observed that in-group love may be correlated with out-group hate: (1) positively, (2) negatively, or (3) zero. In addition, F&T cite several sources that indicate that a number of variables commonly used in cross-cultural psychology are correlated, such as conservatism-liberalism and collectivism-individualism. However, F&T do not report the magnitudes of these correlations. Qualitative expressions like “overlap considerably” or “are similar to” are insufficient to relate these sociocultural constructs psychometrically. Because these terms are afterwards used interchangeably, it would be useful to know exactly how correlated they are, empirically and quantitatively rather than merely theoretically.

This theoretical argument would appear to fit easily within a life history framework, but the way that synthesis might be accomplished remains unclear. F&T acknowledge that parental effort and nepotistic effort are key life history variables (e.g., Figueredo et al. Reference Figueredo, Vásquez, Brumbach and Schneider2007). Religiosity and moral attitudes are also correlates of slow life history strategy (e.g., Figueredo et al. Reference Figueredo, Vásquez, Brumbach and Schneider2007; Gladden et al. Reference Gladden, Welch, Figueredo and Jacobs2009). Furthermore, F&T also note that slow life history strategy has been empirically linked to higher degrees of assortative pairing of both social and romantic partners in a recent cross-cultural study (Figueredo & Wolf Reference Figueredo and Wolf2009). A complicating factor, however, is that another recent cross-cultural study (Andrzejczak et al. Reference Andrzejczak, Jones, Smith, Montero and Figueredo2007; Figueredo et al. Reference Figueredo, Andrezejczak, Jones and Smith-Castro2011a; Jones et al., submitted) found that slow life history is positively predictive of positive ethnocentrism but negatively predictive of negative ethnocentrism. This latter finding does not appear to fit the pattern.

F&T acknowledge that local parasite prevalence is a major force in life history evolution. Indeed, the balance between intrinsic and extrinsic mortality is an important feature in an organism's ecology that, according to life history theory, leads to very specific predictions about behavioral adaptations (see Ellis et al. Reference Ellis, Figueredo, Brumbach and Schlomer2009). Intrinsic morbidity-mortality denotes a threat over which the organism has some control by means of evolvable adaptive responses, such as reallocating resources to buffer or eliminate the threat. Characteristics that may serve in this strategic response include “age, health, size, competitive abilities, metabolism, immune functioning, and related competencies” (Ellis et al. Reference Ellis, Figueredo, Brumbach and Schlomer2009, p. 14). Extrinsic morbidity-mortality denotes a threat that cannot be averted by the organism's attempted countermeasures: An organism may implement behavioral tactics directed towards escaping the threat, but the source of morbidity-mortality is insensitive to these responses. This failure is not due to suboptimal decision-making on the part of the organism. Extrinsic threat is simply unavoidable.

One of the predictions that F&T derive from these life history considerations is that there should be a curvilinear, rather than rectilinear, relationship between parasite prevalence and extrinsic mortality, and hence (indirectly) with assortative sociality. Curiously, though, they only address these with some post hoc comparisons at the end, rather than formally testing the proper curvilinear regression models, which could be readily accomplished with the existing data. Either way, we remain skeptical that mere parasite prevalence is sufficient to select a fast life history strategy (Ellis et al. Reference Ellis, Figueredo, Brumbach and Schlomer2009). Rather, two other factors are paramount: (1) the schedule of age-specific morbidity-mortality, and (2) the temporal stability of these schedules. When systematic variance in juvenile morbidity-mortality is high, it becomes possible to engage in counteracting behaviors, such as investing in competitive ability, to buffer the threats of morbidity-mortality. When threats vary unpredictably across juveniles within a single generation, a strategy that diversifies offspring to partially evade these threats is instead selected. When adults are at greater risk, a strategy of earlier reproduction is selected.

Eppig et al. (Reference Eppig, Fincher and Thornhill2010) recently argued that general cognitive ability is reduced developmentally by parasite stress as a result of a trade-off between investing in two bioenergetically demanding domains: general cognitive functioning and immune functioning. If infectious disease stress can disrupt general cognitive ability developmentally, it may also disrupt executive control, or inhibition of prepotent responses. Some evidence suggests that executive control may inhibit xenophobia (and related features, including disgust sensitivity and in-group preference) (e.g., Gladden Reference Gladden2010; MacDonald et al. Reference MacDonald, Figueredo, Wenner and Howrigan2007). The finding that infectious disease stress disrupts general cognitive functioning also suggests the alternative hypothesis that disease stress may increase ethnocentrism and xenophobia by disrupting executive control as a side effect of a developmental trade-off with investing immune functioning, rather than as an adaptive tactic to guard against pathogenic transmission. Because executive functioning is implicated in general criminality (Gottfredson & Hirschi Reference Gottfredson and Hirschi1990) and interpersonal aggression (Figueredo et al. Reference Figueredo, Gladden, Hohman and Roberts2011b), this alternative mediating mechanism might explain the reported correlation between infectious disease stress and violent aggression across the United States (Thornhill Reference Thornhill2010).

Finally, high disease stress could result in another proposed feature of assortative sociality: philopatry itself. Disease stress could render unhealthy individuals unable to migrate sufficiently long distances to leave their natal group. This alternative hypothesis suggests that the pattern of correlations offered in support of the parasite stress theory of sociality may result as a non-adaptive by-product of infectious disease stress rather than as an adaptive strategy to avoid disease. This alternative side-effect hypothesis needs to be addressed to strengthen the proposed interpretation.