Across multiple measures, we found that worldwide variation in pathogen prevalence substantially predicted societal tendencies towards individualism/collectivism. Within ecological regions characterized by higher prevalence of infectious diseases, human cultures are characterized by greater collectivism. The size of this effect was substantial and remained significant even when controlling statistically for potential confounding variables. The effect also remained strong when broader cultural regions (rather than individual countries or territories) were treated as the units of analysis.
These findings are consistent with the conjecture that, while individualism may confer certain kinds of benefits upon individuals and the societies they create, the behaviours that define individualism may also enhance the likelihood of pathogen transmission, and thus may be functionally maladaptive under conditions in which pathogens are highly prevalent. By contrast, the behaviours that define collectivism may function in the service of antipathogen defence, and thus be especially adaptive under conditions of high pathogen prevalence.
These results complement and substantially extend previous results linking regional variation in pathogen prevalence to the evolution of cross-cultural differences. Sherman & Billing (1999)
suggested a link between pathogens and regional differences in cuisine. Several sets of research results have linked pathogen prevalence to cross-cultural differences in values and norms pertaining to mating and parenting behaviour (Low 1990
; Gangestad & Buss 1993
; Gangestad et al. 2006
; Quinlan 2007
). There is also recent evidence that worldwide variation in pathogen prevalence predicts cultural differences in personality traits, like extraversion (Schaller & Murray in press
). Our results reveal that the predictive effects of pathogen prevalence are not limited to isolated cultural traits; indeed, the effects of pathogen prevalence are observed on a paradigmatic element of culture that, in the eyes of many social scientists, is fundamental to any understanding of cross-cultural differences (Triandis 1995
; Heine 2008
These findings also help to explain additional variables that are correlated with individualism/collectivism. A correlation between individualism/collectivism and latitude has frequently been noted, but never fully explained (Cohen 2001
; Hofstede 2001
; Kashima & Kashima 2003
). Our results imply that this correlation is substantially, albeit not completely, accounted for by pathogen prevalence: the meteorological and ecological conditions associated with lower latitudes provide the ideal circumstances for the proliferation of pathogens (Guernier et al. 2004
), which in turn constrain cultural values towards collectivism. Similarly, there is a correlation between individualism and mean levels of extraversion, which has led to some speculation about the possible causal relations between cultural values and personality traits (Hofstede & McCrae 2004
). Our results, in conjunction with those reported by Schaller & Murray (in press)
, suggest that the correlation between individualism and extraversion is largely spurious, resulting from the fact that both individualism and extraversion are strongly predicted by pathogen prevalence.
Many researchers have observed a strong positive correlation between economic affluence (i.e. GDP per capita
) and individualism and have articulated specific psychological and societal mechanisms through which affluence might lead to individualism (Triandis 1995
; Hofstede 2001
). Our results suggest that the sizeable correlation between affluence and individualism results in part from shared variance with pathogen prevalence. Even the apparently unique effect of GDP per capita
may indirectly reflect some causal role of pathogens, given that infectious diseases are powerful inhibitors of economic development (Sachs & Malaney 2002
). Thus, the extant literature on individualism/collectivism may overestimate economic influences, while underestimating the causal influence of pathogens.
It follows from our analysis that pathogen prevalence may also predict additional cross-cultural differences that are yet to be investigated. If the effects on individualism/collectivism result in part from the antipathogenic consequences of conformity, then explicit behavioural indicators of conformity may be predicted by pathogen prevalence. Pathogen prevalence may also explain cultural differences in ideological tendencies, such as authoritarianism and political conservatism (e.g. Thornhill & Fincher 2007
). And it may predict cross-cultural differences in practices pertaining to learning and education: where pathogens are prevalent, cultures are likely to encourage modes of learning that emphasize imitation and emulation of prestigious in-group members (whereas in less pathogenic environments, there may be greater encouragement for individual experimentation and trial-and-error learning). Pathogen prevalence may also predict cross-cultural variation in other characteristically collectivistic behaviours, such as extended nepotism and in-group care more generally. Existing cross-cultural analyses provide limited evidence consistent with some of these hypotheses (e.g. Bond & Smith 1996
; Georgas et al. 2001
), but rigorous empirical tests have yet to be conducted.
It will also be important for future research to determine the mechanism(s) through which regional variability in pathogen prevalence produces cultural variability along the individualism/collectivism dimension. At least three different kinds of mechanisms can be envisaged; they are not mutually exclusive and may coexist. One is that of cultural transmission. Among humans, culturally specific cognitive and behavioural tendencies can emerge over time as a consequence of local ecological pressures on the information that individuals learn from and teach each other (Richerson & Boyd 2005
). For instance, in regions characterized by high pathogen prevalence, individuals may make deliberate efforts to encourage others to adopt collectivistic (rather than individualistic) behavioural tendencies. Regional differences in individualism/collectivism might also have emerged through locally adaptive allelic differences. As with many other attitudes and behavioural dispositions, individual tendencies towards individualism or collectivism are likely to be substantially heritable (Bouchard & McGue 2003
). It is possible that in regions characterized by a high level of pathogen prevalence, there has been a selection process favouring alleles probabilistically associated with collectivism (whereas alleles associated with individualism may be relatively favoured in regions with low prevalence of pathogens). We suspect that a different kind of genetic adaptation might also be at work. Because individual tendencies towards either individualism or collectivism may confer either fitness costs or benefits, depending on ecological circumstances, some of the genetic and associated developmental substrates for these tendencies may be characterized by a species-typical, evolved sensitivity to informational inputs from the immediate environment—including input indicating the prevalence of pathogens.