A Paris suburb, on the eve of the French Revolution. The shift to democracy and individualism began under conditions of high pathogen prevalence and long before modern sanitation (source)
Is stress from parasites a major cause of psychological differences among humans? Yes, if we are to believe a popular theory in evolutionary psychology. According to this theory, when people develop in a parasite-infested environment, they behave in a way that reduces their likelihood of infection. They become less curious, less exploratory, and less open to strangers. The result is a cultural system that is less conducive to learning, openness, and tolerance—in short, what we like to call progressive values.
As Thornhill et al. (2010) argue:
[…] the predictions of the parasite-stress model are consistent with the marked increase in the liberalization of social values that began to occur in the West in the 1960s and 1970s (e.g., civil rights, women’s rights, gay and lesbian rights, anti-authoritarianism, etc.). In the West, but not outside of it, infectious-disease prevalence was reduced dramatically a generation or two earlier as a result of widespread availability of antibiotics, child vaccination programs, food- and water-safety practices, increased sanitation and vector control.
This theory is used to explain not only cultural differences over time, but also cultural differences over space, i.e., between different human populations:
The parasite-stress model of human sociality provides an evolutionarily informed explanation of why specific human populations inhabiting different parts of the planet (northern Europe versus southern Europe, for instance) are often described by different traits, different values, and different cultural norms.
[…] parasite prevalence also is expected to predict other forms of political liberalism. For example, democratization is accompanied by the liberation of women from the tradition of masculine social control, which manifests in an increase in women’s civil rights and political representation (Inglehart, 2003; Wejnert, 2005; Welzel, 2007). It follows from the parasite-stress model that this form of liberalism should be more pronounced within populations that have a relatively low prevalence of parasites. It is. Across many countries of the world, parasite prevalence correlates negatively with national indicators of gender equality (Thornhill et al., 2010)
Several objections come to mind. Liberalism goes back long before the 1960s. Think of the American and French revolutions. Think of the abolitionists, the chartists, and the suffragettes. These were genuine mass movements that caught the imagination of ordinary people, and not just the elites. Yet they occurred at a time when young men and women routinely died from pathogens under conditions like those of the developing world today. And those conditions persisted well into the 20th century. It really wasn’t until the interwar years that doctors began to cure more people than they killed.
The parasite-stress model has been re-examined by Hackman and Hruschka (2013) with respect to the United States. They confirm that pathogen prevalence correlates with collectivism, strength of family ties, homicide, child maltreatment, and religious commitment. These correlations, however, hold true only for sexually transmitted diseases. Non-STD infections show no correlation with the above behaviors. Moreover, the STD correlation may simply be a side effect of lifestyle choices. As the authors note: “A life history model can explain these ambiguous results by treating STDs as an outcome of faster life history strategies rather than a driver of behavioral adaptations.” Indeed, the data are best explained by two variables: early childbirth and race, i.e., non-Hispanic white American, Hispanic American, or black American:
Our two-component measure [early childbirth and race] showed that across race categories, teenage birth rates are predictive of three-generation households and proportion of the population living alone. We conclude that these findings are inconsistent with the PST [pathogen-stress theory], but fit well with an alternative model based on life history allocations. (Hackman and Hruschka, 2013)
Parasite-stress theory reverses cause and effect. Pathogens are less prevalent in those human populations that have integrated principles of modern hygiene into their lives. Those same populations have also adopted other aspects of behavioral modernity—pacifism, individualism, reduced importance of kinship, etc. More broadly speaking, the construction of freer, more open societies cannot happen without certain psychological predispositions: first, higher anger thresholds and less willingness to use violence as a means to settle personal disputes; second, a time orientation that allocates more resources to the future and fewer to the present. Evidently, if you’re more oriented to the future, you’ll avoid choices that may lead to illness and early death.
This point may seem obvious, yet it’s surprising how unobvious it seems to some people, especially those, like evolutionary psychologists, who should know better. How come? Keep in mind that elite approval is necessary for advancement in society, particularly for academics who work amidst offspring of the elite and who help legitimize the dominant social agenda. To gain acceptance for their own pet ideas, academics unconsciously, or consciously, sign on to the elite's agenda.
As Thornhill et al. (2010) note: “[…] public health initiatives are most likely to have additional consequences for societies (e.g., promotion of civil liberties and egalitarian value systems).” Here, the academic is no longer pretending to be a disinterested observer. The role is more like that of a cheerleader … or worse.
Barkow, J.H., Cosmides, L., & Tooby, J. (eds.) (1992). The Adapted Mind. Evolutionary Psychology and the Generation of Culture, New York, Oxford: Oxford University Press.
Hackman, J. and D. Hruschka. (2013). Fast life histories, not pathogens, account for state-level variation in homicide, child maltreatment, and family ties in the U.S., Evolution and Human Behavior, 34, 118-124.
Thornhill, R., C.L. Fincher, D.R. Murray, and M. Schaller. (2010). Zoonotic and Non-Zoonotic Diseases in Relation to Human Personality and Societal Values: Support for the Parasite-Stress Model, Evolutionary Psychology, 8, 151-169.http://www.epjournal.net/wp-content/uploads/EP08151169.pdf