Van de Vliert's target article notes that cold and hot conditions pose divergent problems, which entail unique psychobehavioral adaptations. Exactly what these divergent problems are, and the unique psychological and cultural adaptations they are likely to cause, deserve further exploration. Distinct logical hypotheses predict that hot and cold climates will lead to convergent cultural adaptations for some of the freedoms that Van de Vliert discusses, and to divergent cultural adaptations for others.

The ancestors of modern humans evolved largely in equatorial regions, which consisted of both temperate and “hot” climates. It is most likely that only the control of fire allowed humans to expand to colder climatic zones (e.g., Burton Reference Burton2009). The adaptive problems posed by very hot and very cold temperatures thus vary in their immediacy. The threat posed by cold temperatures is much more direct and immediate than the threat posed by heat; void of thermal technology, a very cold climate is a much more immediate threat to survival than a very hot one.

Hot climates, on the other hand, pose a suite of indirect adaptive challenges. One substantial challenge to survival in hot climates is not heat itself but infectious diseases, which proliferate in hotter climates (Epstein Reference Epstein1999). Infectious diseases have likely caused more deaths than predators, natural disasters, wars, and noninfectious diseases combined (e.g., Inhorn & Brown Reference Inhorn and Brown1990) and have been a key selective force in the evolution of human physiology and culture (Armelagos & Dewey Reference Armelagos and Dewey1970; Black Reference Black1975).

The causes of mortality in traditional societies reflect the differential threats posed by hot versus cold weather: whereas infectious disease causes more than half the deaths in traditional post-contact equatorial cultures (e.g., Gurven & Kaplan Reference Gurven and Kaplan2007), leading causes of death among circumpolar traditional cultures (such as the Inuit of Canada or the Sámi of Scandinavia) involve noninfectious ailments, many of which may be the result of diet (Hassler et al. Reference Hassler, Johansson, Sjölander, Grönberg and Damber2005; Peters Reference Peters2010). The different adaptive problems that drive cultural and genetic evolution in these disparate climates suggest divergent logical predictions about how these climates affect certain freedoms.

Let us first consider the utility of conformity (low “freedom of expression”) in hot climates. Whereas unequivocal conformity to cultural norms can have costs (e.g., it inhibits innovation), it has antipathogen benefits: many cultural prescriptions (especially those pertaining to hygiene, food, and sex) serve to buffer against pathogen transmission (e.g., Fabrega Reference Fabrega1997). These antipathogen benefits should be especially likely to outweigh the costs of conformity when the threat of disease is especially high. The implication is that the resultant higher disease threat in hotter climates should negatively predict freedom of expression. Several lines of research support this hypothesis. Conformity and obedience are highest in cultures characterized by higher disease threat (controlling for monetary resources), higher for people who are dispositionally more worried about the threat of disease, and higher when disease threat (but not non-disease threat) is temporarily perceptually salient (Murray & Schaller Reference Murray and Schaller2012; Murray et al. Reference Murray, Trudeau and Schaller2011; Wu & Chang Reference Wu and Chang2012). Therefore, disease threat may mediate the relationship between hot climates and lower freedom of expression (at least in countries lacking monetary resources to buffer against this threat).

Just as some cultural norms buffer against harm from pathogens, other cultural norms may buffer against the immediate threats inherent in very cold environments. Deviating from established norms in cold environments could pose immediate challenges to survival (e.g., norms pertaining to methods of thermal protection or to fire preparation and maintenance). To date no study has uniquely investigated the impacts of cold-based threats on conformity. However, despite the different adaptive challenges posed by these climates, the logical predictions converge for their cultural consequences: freedom of expression should be especially costly in both hot and cold (low monetary resource) environments.

Ethnocentrism and xenophobia (“freedom from discrimination”), on the other hand, may produce divergent predictions between different extreme climates. Disease threat has implications for xenophobia: The immune system is calibrated to pathogens common within one's ingroup, and contact with non-ingroup members increases the probability of exposure to novel pathogens. Drawing on a cost/benefit framework, the implication is that xenophobia is more beneficial – and thus more prevalent – when pathogen threat is especially high (e.g., in hotter climates). Indeed, group assimilation is negatively predicted, and xenophobia positively predicted, by higher disease threat (Fincher & Thornhill Reference Fincher and Thornhill2008; Schaller & Murray Reference Schaller, Murray, Schaller, Norenzayan, Heine, Yamagishi and Kameda2010). Xenophobia is also higher when disease threat is perceptually salient, and when the immune system is temporarily compromised (Faulkner et al. Reference Faulkner, Schaller, Park and Duncan2004; Navarrete et al. Reference Navarrete, Fessler and Eng2007). Therefore, the covariation between demanding hot climates and lower freedom from discrimination is likely to be driven, at least in part, by variation in disease threat.

Predicting the relationship between more demanding cold climates and xenophobia is less clear. There are still benefits of xenophobia in very cold climates (e.g., lower probability of interactions with those who are more likely to violate local norms, Kurzban & Leary Reference Kurzban and Leary2001); however, the benefits of outgroup contact in cold climates (trade and better resource stability, large-scale cooperation) may outweigh the costs in these low-disease environments. Therefore, the opposite prediction that more demanding cold climates predict lower xenophobia is equally feasible, and demanding hot and cold climates make divergent predictions for freedom from discrimination.

The feasibility of analyses investigating the interrelationships between disease, climate, and monetary resources is constrained by the strong relationship between temperature and wealth; cold rich countries far outnumber cold poor ones, and hot poor countries far outnumber hot rich ones. This relationship is also at least partly attributable to variation in disease, given the bidirectional causal relationship between disease prevalence and wealth (e.g., Gallup & Sachs Reference Gallup and Sachs2001). As the target article notes, single-factor explanations of culture are myopic; in fact, the state of the literature is now such that we can extend beyond two-factor explanations as well. The next step in this research will be to investigate and test structural models of the interrelationships between causal ecological variables and their impact on the evolution of cultural differences.