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.

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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.

Case in point: the United States. Varnum and a colleague, Igor Grossmann, reported Monday in the journal Nature Human Behavior, that a decrease in the prevalence of this country's nine biggest infectious diseases over the past 70 years has been accompanied by a dramatic upswing in gender equality.

“It's one of the strongest relationships I’ve ever observed in my work,” said Varnum, an assistant professor at Arizona State University. He compares the statistical correlation between low pathogen loads and gender equality to the link between smoking and lung cancer.

The notion that behavior is influenced by disease rates, termed “parasite stress theory,” has its roots in biology. Numerous studies have shown that animals have evolved to behave in ways that limit their exposure to pathogens. Wild mice quarantine themselves when they are sick. Infected ants leave their nests to die. Some primates ostracize ailing members of their group to prevent disease from spreading.

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In 2012, researchers Corey Fincher and Randy Thornhill published a paper in the journal Behavioral and Brain Sciences arguing that the theory explains broad cultural changes in human societies. They found stronger family ties and heightened religiosity in societies with greater parasite loads, because, as they put it, sticking to one's own group “is more important for the avoidance of infection from novel parasites and for the management of infection in regions with high levels of parasite stress.”

That idea is still contentious, and sociologists have argued that the relationship between pathogen prevalence and human behavior is more nuanced, or even inverted. A 2013 study of U.S. data suggested that only STDs, rather than all infectious diseases, affected behavior like interpersonal violence, child maltreatment and religious adherence. The study also found that the effect vanished when the researchers controlled for rapid life histories (biology jargon for how rapidly creatures grow up, mate and reproduce) — suggesting that the pivotal factor is the age at which people are marrying and having children, not how many germs they're carrying.

Varnum and Grossmann note that their study doesn't prove lower parasite load directly increases gender equality. It only demonstrates that there is a very strong correlation.

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The pair looked at four environmental factors — resource scarcity, warfare, climatic stress and infectious disease — that could affect women's rights. Varnum explained that anxiety about poverty, violence or severe weather potentially causing an early death could also prompt people to marry and reproduce earlier, which tends to disadvantage women.

They then quantified each of those factors and compared the numbers with an index of gender equality, which was calculated based on the size of the wage gap, female representation in Congress, references to female pronouns in news reports and literature, and attitudes about women's rights as measured by Gallup polls. Statistical analysis revealed that declining infectious disease rates had by far the strongest correlation with women's advancement, even when the researchers controlled for influences such as overall collectivism and conservatism. Factoring in lower teen birthrates somewhat weakened the correlation, suggesting that slower life-history strategies independently boost equality.

What's more, the decline in disease rates appeared to predict an increase in women's rights: “The links are stronger for pathogens at a particular time predicting gender equality in the future, rather than the other way around,” Varnum said.

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The researchers tested their model by comparing pathogen prevalence and gender equality in Britain. The correlation there held strong.

“Ideally we’d love to get data from a number of other societies around the world,” Varnum said, noting that this will be difficult in places with less comprehensive public health and political data. “I would be interested to see how this relationship looks across the globe over time.”