The debate on cultural group selection (CGS) would be more fruitful if it considered the role of human ecology as a critical moderator of CGS. In particular, we argue that ecological and historical threats to human societies strengthen CGS pressures. As we summarize below, there is substantial evidence from field, experimental, computational, and genetic studies suggesting that exposure to threat increases the strength of social norms and punishment of deviance, which are conditions that Richerson et al. identify as important mechanisms that maintain intergroup variation. Hence, ecological and historical threats can critically moderate the strength of CGS pressures and may provide insight into precisely where, when, and to what extent CGS occurs.

Field research across 33 nations first documented the connection between ecological and historical threat and the degree of norm strength in groups (Gelfand et al. Reference Gelfand, Raver, Nishii, Leslie, Lun, Lim, Duan, Almaliach, Ang, Arnadottir, Aycan, Boehnke, Boski, Cabecinhas, Chan, Chhokar, D'Amato, Ferrer, Fischlmayr, Fischer, Fulop, Georgas, Kashima, Kashima, Kim, Lempereur, Marquez, Othman, Overlaet, Panagiotopoulou, Peltzer, Perez-Florizno, Ponomarenko, Realo, Schei, Schmitt, Smith, Soomro, Szabo, Taveesin, Toyama, Van de Vliert, Vohra, Ward and Yamaguchi2011). Ecological and human-made threats were theorized to increase the need for clear rules and coordination for the purpose of survival – whether it is to reduce chaos in nations that have high population density, to effectively deal with resource scarcity, to coordinate in the face of natural disasters, to defend against territorial threats, and/or to contain the spread of infectious disease. Societies facing these ecological and human challenges therefore develop strong norms and sanctions for deviance (i.e., are tight) to enhance coordination and deal effectively with such threats. By contrast, societies with less threat have a lower need for coordination, and hence are loose, that is, have weaker norms and much higher tolerance for deviant behavior. Using archival and survey data, we tested this hypothesis across a wide range of variables and found that societies facing ecological and historical threats do indeed develop stronger norms and punishment of deviance (Gelfand et al. Reference Gelfand, Raver, Nishii, Leslie, Lun, Lim, Duan, Almaliach, Ang, Arnadottir, Aycan, Boehnke, Boski, Cabecinhas, Chan, Chhokar, D'Amato, Ferrer, Fischlmayr, Fischer, Fulop, Georgas, Kashima, Kashima, Kim, Lempereur, Marquez, Othman, Overlaet, Panagiotopoulou, Peltzer, Perez-Florizno, Ponomarenko, Realo, Schei, Schmitt, Smith, Soomro, Szabo, Taveesin, Toyama, Van de Vliert, Vohra, Ward and Yamaguchi2011).

Further research confirmed this relationship at the state level in the United States. Harrington and Gelfand (Reference Harrington and Gelfand2014) found that tighter states (e.g., those which have strong norms and punishment of deviance) have a greater degree of natural disasters and environmental vulnerabilities, fewer resources (indicated by greater rates of food insecurity), a greater incidence of disease and health vulnerabilities, and perceive a greater degree of external threat, reflected in the desire for more national defense spending and greater rates of military recruitment, as compared to loose states. (i.e., those which have weaker norms and less punishment of deviance). Individuals in tighter states also had higher levels of conscientiousness – a personality dimension that reflects greater impulse control, cautiousness, desire for orderliness, and conformity to norms – and lower openness – a personality dimension that reflects non-traditional values and beliefs, interest and curiosity toward new ideas, and tolerance for other cultures (John et al. Reference John, Naumann and Soto2008). Tighter states also had significantly lower residential mobility and supported more isolationist policies (such as buying American products exclusively and supporting government restriction of imported products) compared to loose states.

Beyond these correlational results, evolutionary game theoretical (EGT) models show that groups that face a high degree of threat require stronger norms and greater punishment of deviance in order to survive (Roos et al. Reference Roos, Gelfand, Nau and Lun2015). Following existing EGT models (Hilbe & Traulsen Reference Hilbe and Traulsen2012; Roos et al. Reference Roos, Gelfand, Nau and Carr2014), we examined the evolution of strategies for cooperation as well as strategies for a subsequent punishment phase in cooperation and coordination games. We found that exposing populations to higher degrees of threat – implemented through lower base-rate payoffs – led to an increase in the survival of agents that adhered to the cooperation or coordination norm and those that punished norm-deviating behaviors. Under low threat, the evolutionary pressures resulted in a more diverse mix of all possible strategies. Besides showing that strong norms and punishment of deviance emerges under chronic levels of high threat, these models also found that temporary increases in threat cause norm strength to increase until the threat subsides. Given that conformity and punishment are important pressures for CGS (per Richerson et al.), it follows that increases in ecological and historical threat contribute to the conditions for CGS to occur.

Our electroencephalography work has also found neurobiological differences in the detection of deviant behaviors among tight and loose groups (Mu et al. Reference Mu, Kitayama, Han and Gelfand2015). Chinese samples, which reported higher degrees of territorial threats, had stronger neurological responses (e.g., greater N400) in the frontal region when viewing social norms violations (e.g., Bob is in the library, he is shouting as compared to Bob is in the library he is studying) in comparison with U.S. samples. Moreover, stronger N400 responses mediated cultural differences in a number of attitudes and behaviors, including higher self-control and ethnocentrism, but lower creativity, among Chinese as compared to U.S. groups. More generally, pronounced neural reactions to norm violations should enable groups to enforce social norms and punish deviant behavior, which, according Richerson et al., should further enhance intragroup cohesion and intergroup variation.

Relatedly, using genetic data, Mrazek et al. (Reference Mrazek, Chiao, Blizinsky, Lun and Gelfand2013) argue that higher frequencies of short (S) allele in the 5-HTTLPR polymorphism of the serotonin transporter gene (SLC6A4) persist in regions of the world prone to ecological threat because heightened threat sensitivity may be adaptive in such regions. Because the S allele has been associated with more aversive reactions to moral violations in groups, they suggest that threat-driven genetic variance in the S allele may underlie differences in moral attitudes across cultures. Their path model linking ecological threat, genetic variability of the S allele, the strength of social norms, and reactions to moral violations provides plausible evidence for this argument.

In sum, as scholars have rushed to either prove or refute cultural group selection, few have explored it as a phenomenon that is modified by the ecological and historical contexts in which human groups are embedded. Considering the role of societal threat in theories of CGS could help resolve the debate of where, to what extent, and how CGS occurs.