by mariah quintanilla

donald trump is president-elect of the united states, and a popular question these past few weeks has been, “how did we get here?” while the media have done some collective soul-searching in an attempt to answer this question, one possibility that we’ve all failed to recognize is biology.

psychologists and biologists have discovered that conservative or liberal ideologies can be more or less predicted in people by their responses to—wait for it—infectious disease and parasites.

in a study released in september in the proceedings of the national academy of sciences, psychologists found that areas with a high persistence of parasites and pathogens correlated with people who hold more conservative views. liberal views, on the other hand, are more prevalent in areas with less disease and fewer parasites. the goal of the study was to tease apart subtle biological and evolutionary factors that shape conservative or liberal human nature across countries, as well as across regions within countries.

how do pathogens influence human behavior?

as humans evolved over time, they learned how to avoid contact with parasites and disease by reacting to certain cues, said joshua tybur, psychology professor at vrije universiteit amsterdam, who led the study.

some reactions to cues might be as simple as avoiding someone’s cough, or feeling disgusted by excessive sweat or moldy bread. all of the conscious or subconscious measures we take to avoid getting sick are part of what scientists call our behavioral immune system, said tybur in a phone interview.

different pathogens and diseases are either more prevalent in certain areas or have existed there longer, which allow people living in those areas to develop immune responses against those pathogens. the spanish demonstrated this when they invaded the new world, inadvertently killing millions of native people by spreading diseases such as smallpox and influenza. the native people died by the masses because they lacked immune defenses against these old-world diseases.

the researchers found that countries with higher infectious disease burdens—like india, china and brazil—were more traditional overall. tybur and his colleagues define traditionalism as “an aspect of conservatism especially related to adherence to group norms.” the u.s. is low on the scale of traditionalism and parasite stress, which some may find surprising considering this year’s election returns.

of the 30 countries surveyed, denmark and sweden demonstrated the least amount of parasite stress and are the least traditional.

a controversial theory

the study is based on the principles of parasite-stress theory, a somewhat controversial theory that says areas with more parasites and infectious disease correlate with conservative human behaviors and ideologies. the theory was first conceived in 2014 by randy thornhill, a biologist at the university of new mexico, and corey fincher, a psychologist at the university of warwick in the u.k.

the theory suggests that “individuals within environments of high infectious disease will acquire conservative values, and individuals within environments of low infectious disease will acquire liberal values,” said thornhill in a phone interview.

opponents of thornhill’s theory point out obvious cultural factors and historical hiccups (like liberalism in times of widespread disease). thornhill and tybur argue that studying the evolutionary mechanisms behind why humans act the way they do in society only creates a more complete and nuanced picture of our species. tybur stressed that his research does not override the myriad modern human behavior that we see today.

“disgust is an emotion that protects us against infectious disease,” thornhill said. disease and parasites can also cause people to avoid those who are different from them, he added.

donald trump, however, is not xenophobic because of parasites and disease, tybur said. someone like vice president-elect mike pence, with morals that are tethered in religious ideology, might demonstrate stronger pathogen-avoidance measures.

the effect of pathogen avoidance on our behavioral immune system probably has little to do with anti-immigrant or anti refugee sentiment, he said, though thornhill disagrees.

“these data explain a small amount of ideology in our study. we don’t want to convey that this is the reason that people have conservative or liberal ideologies,” tybur said.