Wouldn’t it be interesting if our human psychology could be altered by the evolutionary implications of infectious diseases our ancestors encountered? Well, that’s actually a real theory!
The Parasite-Stress Model was first suggested by researchers Corey Fincher and Randy Thornhill; this theory of human evolution proposes that parasites, and other zoonotic diseases, encountered by a species can shape the development of that species' social behavior. The differences in how parasites and diseases stress people's development is what leads to differences in their biological mate value and mate preferences, as well as differences across culture.
The Parasite-Stress Model of human sociality implies that parasitic diseases have a general effect in shaping major features of human psychology and behavior. Throughout evolutionary history, our ancestors faced physical problems as a result of infectious diseases. In addition to the classical immune system being compromised, this theory suggests that other mechanisms come into play—such as the behavioral immune system—that facilitate behavioral avoidance of parasite transmission in the first place. The behavioral immune system can manifest through cognitive responses to local levels of parasite stress, which can vary by geographic region.
Global data on human populations support the idea that in countries with a higher prevalence of infectious diseases, people tend to have greater sexual restrictedness, as unrestricted sexual behavior can lead to disease transmission. However, it should be noted that these cultures are defined also by more conservative and traditional values (Schaller and Murray, 2008; Thornhill, Fincher, and Aran, 2009). This cross-national positive relationship between the prevalence of infectious disease and sexual restrictiveness is stronger for women than for men.
This same logic applies to other human behavioral tendencies, including extraversion. Certain populations may generally be more keen of larger social networks, including mating pools, but that also implies greater exposure to infectious diseases (Nettle, 2005). With the support of cross-national data describing the personality traits of tens of thousands of people, it has been found that human populations are likely to be characterized by extraversion primarily under ecological conditions of low parasite prevalence, whereas a more introverted personality style is more likely to emerge when the prevalence of parasites is high (Schaller and Murray, 2008).
The Parasite-Stress Model is thought to have much broader implications than just human behavior. According to one study, it follows from the parasite-stress model that political liberalism should be more pronounced within populations that have a relatively low prevalence of parasites because the liberation of women from the tradition of masculine social control, a liberal idea, would make sense with the basis of the theory (Inglehart, 2003; Wejnert, 2005; Welzel, 2007).
But there are so many other factors, like income inequality and climate change, that could possibly influence human behavior that you can’t just point to the Parasite-Stress Model as the sole explanation of our behavior (with the data we currently have, anyway). In order to have more definitive answers and to assure that this relationship is not merely an artifact of statistical non-independence, we would need to compute parasite-prevalence, personality traits, and societal value scores for larger culturally-distinct world regions (Murdock, 1949).
Various ways to measure differences in human behavior across populations include:
The Female Sociosexual Orientation Test, a self-report used to assess a disposition toward a certain sexuality
The Extraversion Test, a questionnaire to evaluate one’s level of extraversion.
The Openness to Experience Test, which rates one’s open-mindedness.
This theory is very interesting, and I hope to see more data in the future. What do you think of this model? Let us know in the comments!