Fincher & Thornhill (F&T) present a strong argument for the primacy of host-parasite coevolutionary races as causal drivers of the geographic distribution of religiosity and family ties. As evolutionary anthropologists interested in both how and why religion does what it does, we applaud this work as a truly biosocial approach to religion. The claim that socially learned and cultural behavior is geared toward dealing with parasite invasions connects proximate- and ultimate-level explanations for religion and can potentially inform neuroscientific, cognitive, and behavioral studies. We focus our comments on costly signaling, warfare, and economic interrelationships.
F&T argue that parasite sociality and evolutionary signaling models complement each other. In areas of high parasite stress, individuals who signal religious commitment reap the benefits from an enhanced barrier that screens out-group members, and from increased support and care in the case of infection. Evolutionary signaling models of religion propose that religious groups achieve efficiency in cooperation through the deterrence of free-riders who would reduce it (Bulbulia Reference Bulbulia2004; Irons Reference Irons and Nesse2001; Sosis Reference Sosis2003; Sosis & Alcorta Reference Sosis and Alcorta2003). What stops potential free-riders from exploiting membership in these groups? F&T's framework provides an answer for both out-group individuals and potential free-riders within the group. Out-group members can be prevented from joining religious groups by being subjected to the contempt and disgust that violations of local norms and the risk of infection generate in existing members. This may be one area where the supernatural content of religious beliefs provides a unique ability to achieve this ostracism by casting out-group individuals as animalistic, demonic, or evil subhuman beings (Hansen & Norenzayan Reference Hansen, Norenzayan and McNamara2006).
On the other hand, even when the costs of religious participation are the same across individuals, within-group differences in immunocompetence may produce a situation where the benefits for signaling are greater for individuals who are more susceptible to disease. Indeed, we regularly find that religion figures prominently in conceptions of illness and healing practices (Reynolds & Tanner Reference Reynolds and Tanner1995, pp. 237–66). Receivers of these signals would benefit from knowing who is the least likely to interact with out-group members who may transmit novel pathogens. Individuals who are better able to balance the risk of infection with the benefits of interaction with out-group members would be expected to signal religious participation at lower levels. In this way, the extent of intergroup interaction may depend on the level of heterogeneity in immunocompetence within groups. This would be an intriguing avenue for empirical study, especially in traditional societies.
However, there may be a tension between the need to avoid pathogens and religious signaling practices. The most effective signals – the ones that are least likely to be faked by low-commitment individuals – may in fact involve opening the body in order to sacrifice blood or body parts, ingesting toxic substances, engaging in extreme physical exertion, or otherwise potentially compromising one's immune system. There is some evidence that scarification and its placement are related to pathogen stress (Singh & Bronstad Reference Singh and Bronstad1997). It would be interesting to investigate how signaling takes different forms in the areas of highest and lowest parasite prevalence to determine how this tradeoff is managed.
A potentially important piece of this argument is how warfare interacts with in-group assortative sociality and parasite stress. For example, signaling behaviors are utilized cross-culturally in solving collective action problems related to intra- and inter-group warfare (Sosis et al. Reference Sosis, Kress and Boster2007). War requires a degree of xenophobia to rationalize hostility to out-groups, and competing developmental models include pathogen stress and risk of resource failure in order to explain socialization into violence (see Ember & Ember Reference Ember and Ember1992; Reference Ember and Ember2007). It may be possible to make predictions about culturally mediated behaviors toward out-groups based on the interaction between parasite stress and temporally varying resource stress. Cashdan's (Reference Cashdan2001) results suggest that xenophobia and ethnocentrism are independent phenomena correlated with violence (both within and across groups) and resource stress, respectively. Where parasite stress is high and neighboring groups have a history of conflict, resource stress would be unlikely to relax these tensions and motivate intergroup trade. Even between groups with a history of relative peace, F&T's argument predicts that during times of famine the group with relative plenty should only risk infection to exchange with stressed groups if they have a large probability of experiencing a future catastrophic shortfall of their own, or if they can greatly exploit the other group in the process. Exactly how contagion risk trades off against economic benefit is an issue that deserves empirical study.
While religion does indeed circumscribe social relations and often limits social contact with other groups, religions are diverse, and some traditions may increase rather than limit exposure to individuals from other geographic locations due to economic reasons. Proselytizing, for example, likely increases exposure to strangers. Thus, a further test of the parasite-stress theory of sociality would assess whether parasite stress and the degree of proselytizing within a religion are negatively related. Aside from proselytizing, religious groups often fill important economic niches by establishing trade relations with coreligionists in distant lands, thereby increasing exposure between individuals who have presumably adapted to different disease environments. It has been argued, for example, that the spread of certain religions, such as Islam throughout Africa, is partially a result of the benefits that shared religious identity has among traders who have little reputational information on each other because of their geographic separation (Ensminger Reference Ensminger1997; Sosis Reference Sosis2005). Then again, in some contexts, economic relations maintain religious pluralism. The religiously pluralistic Silk Road in Eurasia, for instance, consisted of remarkably expansive trade networks of regularly interacting individuals of many religious traditions from various ethnic groups, including Buddhists, Manicheans, Muslims, Zoroastrians, shamanists, and animists (Beckwith Reference Beckwith2009; Foltz Reference Foltz1999). Presumably, pathogen stress was significantly higher at this time than at present. To further evaluate the parasite-stress theory of sociality, future work should examine the differences across religions in their social and economic interactions with outsiders.
Fincher & Thornhill (F&T) present a strong argument for the primacy of host-parasite coevolutionary races as causal drivers of the geographic distribution of religiosity and family ties. As evolutionary anthropologists interested in both how and why religion does what it does, we applaud this work as a truly biosocial approach to religion. The claim that socially learned and cultural behavior is geared toward dealing with parasite invasions connects proximate- and ultimate-level explanations for religion and can potentially inform neuroscientific, cognitive, and behavioral studies. We focus our comments on costly signaling, warfare, and economic interrelationships.
F&T argue that parasite sociality and evolutionary signaling models complement each other. In areas of high parasite stress, individuals who signal religious commitment reap the benefits from an enhanced barrier that screens out-group members, and from increased support and care in the case of infection. Evolutionary signaling models of religion propose that religious groups achieve efficiency in cooperation through the deterrence of free-riders who would reduce it (Bulbulia Reference Bulbulia2004; Irons Reference Irons and Nesse2001; Sosis Reference Sosis2003; Sosis & Alcorta Reference Sosis and Alcorta2003). What stops potential free-riders from exploiting membership in these groups? F&T's framework provides an answer for both out-group individuals and potential free-riders within the group. Out-group members can be prevented from joining religious groups by being subjected to the contempt and disgust that violations of local norms and the risk of infection generate in existing members. This may be one area where the supernatural content of religious beliefs provides a unique ability to achieve this ostracism by casting out-group individuals as animalistic, demonic, or evil subhuman beings (Hansen & Norenzayan Reference Hansen, Norenzayan and McNamara2006).
On the other hand, even when the costs of religious participation are the same across individuals, within-group differences in immunocompetence may produce a situation where the benefits for signaling are greater for individuals who are more susceptible to disease. Indeed, we regularly find that religion figures prominently in conceptions of illness and healing practices (Reynolds & Tanner Reference Reynolds and Tanner1995, pp. 237–66). Receivers of these signals would benefit from knowing who is the least likely to interact with out-group members who may transmit novel pathogens. Individuals who are better able to balance the risk of infection with the benefits of interaction with out-group members would be expected to signal religious participation at lower levels. In this way, the extent of intergroup interaction may depend on the level of heterogeneity in immunocompetence within groups. This would be an intriguing avenue for empirical study, especially in traditional societies.
However, there may be a tension between the need to avoid pathogens and religious signaling practices. The most effective signals – the ones that are least likely to be faked by low-commitment individuals – may in fact involve opening the body in order to sacrifice blood or body parts, ingesting toxic substances, engaging in extreme physical exertion, or otherwise potentially compromising one's immune system. There is some evidence that scarification and its placement are related to pathogen stress (Singh & Bronstad Reference Singh and Bronstad1997). It would be interesting to investigate how signaling takes different forms in the areas of highest and lowest parasite prevalence to determine how this tradeoff is managed.
A potentially important piece of this argument is how warfare interacts with in-group assortative sociality and parasite stress. For example, signaling behaviors are utilized cross-culturally in solving collective action problems related to intra- and inter-group warfare (Sosis et al. Reference Sosis, Kress and Boster2007). War requires a degree of xenophobia to rationalize hostility to out-groups, and competing developmental models include pathogen stress and risk of resource failure in order to explain socialization into violence (see Ember & Ember Reference Ember and Ember1992; Reference Ember and Ember2007). It may be possible to make predictions about culturally mediated behaviors toward out-groups based on the interaction between parasite stress and temporally varying resource stress. Cashdan's (Reference Cashdan2001) results suggest that xenophobia and ethnocentrism are independent phenomena correlated with violence (both within and across groups) and resource stress, respectively. Where parasite stress is high and neighboring groups have a history of conflict, resource stress would be unlikely to relax these tensions and motivate intergroup trade. Even between groups with a history of relative peace, F&T's argument predicts that during times of famine the group with relative plenty should only risk infection to exchange with stressed groups if they have a large probability of experiencing a future catastrophic shortfall of their own, or if they can greatly exploit the other group in the process. Exactly how contagion risk trades off against economic benefit is an issue that deserves empirical study.
While religion does indeed circumscribe social relations and often limits social contact with other groups, religions are diverse, and some traditions may increase rather than limit exposure to individuals from other geographic locations due to economic reasons. Proselytizing, for example, likely increases exposure to strangers. Thus, a further test of the parasite-stress theory of sociality would assess whether parasite stress and the degree of proselytizing within a religion are negatively related. Aside from proselytizing, religious groups often fill important economic niches by establishing trade relations with coreligionists in distant lands, thereby increasing exposure between individuals who have presumably adapted to different disease environments. It has been argued, for example, that the spread of certain religions, such as Islam throughout Africa, is partially a result of the benefits that shared religious identity has among traders who have little reputational information on each other because of their geographic separation (Ensminger Reference Ensminger1997; Sosis Reference Sosis2005). Then again, in some contexts, economic relations maintain religious pluralism. The religiously pluralistic Silk Road in Eurasia, for instance, consisted of remarkably expansive trade networks of regularly interacting individuals of many religious traditions from various ethnic groups, including Buddhists, Manicheans, Muslims, Zoroastrians, shamanists, and animists (Beckwith Reference Beckwith2009; Foltz Reference Foltz1999). Presumably, pathogen stress was significantly higher at this time than at present. To further evaluate the parasite-stress theory of sociality, future work should examine the differences across religions in their social and economic interactions with outsiders.