There’s an idea called “parasite stress theory,” which argues that a species’ physical and social traits are shaped by the pathogens it encounters – that the trajectory of humanity, in other words, has been steered in large part by the germs inside our bodies. And in a study published earlier this week in the journal Nature Human Behavior, researchers identified a compelling example of parasite stress theory in practice: strides in gender equality over the past several decades are closely linked to a drop in infectious disease.
For the study, authors Michael Varnum and Igor Grossmann (psychologists from Arizona State University and the University of Waterloo in Canada, respectively) used four factors to measure gender inequality in the United States, pulling data from 1951–2013 on the wage gap, the number of women in Congress, “sexist work attitudes” (determined using Gallup polls on whether workers would prefer a male boss), and “linguistic representation in cultural products” (or the ratio of male to female pronouns in books).
They also looked at four other issues across the same time span with the potential to influence social change: infectious disease, war, resource scarcity, and “climatic stress.” Of those, disease was far and away the most closely linked to an uptick in women’s circumstances — “one of the strongest relationships I’ve ever observed in my work,” Varnum told the Washington Post, which explained why that might be the case:
Suspicion of strangers is a good way to ward off unfamiliar germs to which you may not have immunity. Emphasis on traditional ways of doing things — cooking, cleaning, child rearing and burial practices — could discourage new, unsafe behaviors. Putting off education, exploration or political engagement in favor of settling down and starting a family could be a means of ensuring your genes are passed on before you die of an illness.
But once the specter of infectious disease is lifted, the outlook shifts. It’s safe to become more individualistic, to embrace difference, to celebrate innovation. People gaze into the unknown and see opportunity, not peril. And that’s when change starts to happen.
It’s important to stress that the relationship seems to go one way: “The links are stronger for pathogens at a particular time predicting gender equality in the future, rather than the other way around,” Varnum told the paper.
The study authors found a similar pattern in the U.K., and hope to apply their method to other societies in the future — but in the meantime, they argued, it’s not too early to dismiss its practical value. “These findings also have practical implications of interest to policymakers,” they wrote: “Our results suggest that efforts to reduce infectious diseases, such as vaccinations, free health care, public sanitation and water treatment, might also increase equality among the sexes.”