UO researchers show how disgust evolved as an emotion

Woman reacts to smell of spoiled food

Next time a refrigerator door opens to the smell of rotting uncooked chicken, consider the moment as an odorous encounter with the origin of disgust. That repulsion is linked to an evolved human emotion that helps avoid exposure to something sickening.

In a project that blended anthropology, biology and psychology, a University of Oregon team explored disgust by studying how Ecuador’s indigenous Shuar people, living in communities with differing levels of market integration, respond to revolting things.

The research was detailed in a paper published online Feb. 23 ahead of print in the Proceedings of the National Academy of Sciences.

“From the point of view of evolved psychology, we’ve demonstrated more directly than any previous research that disgust is an evolved emotion that functions to regulate our exposure to pathogens,” said Lawrence Sugiyama, a professor in the Department of Anthropology. “It’s a behavior that is calibrated to account for the relative costs and benefits of avoidance in a particular environment.”

The project, led by former UO doctoral student Tara J. Cepon-Robins, now an assistant professor of anthropology at the University of Colorado, was among many inquiries done in almost 30 years of fieldwork by Sugiyama among indigenous forager-farming populations in Ecuador’s Amazonian region.

The research focused on 75 subjects, ages 5-59, in 28 households in three Shuar communities that varied in their isolation from or partial involvement in the larger market economy. Examined were measurements of disgust sensitivity to sources of infection and levels of bacterial, viral and parasitic infections from blood and fecal samples collected in the UO-led Shuar Health and Life History Project.

While data related to exposure to abundant parasitic worms in Shuar communities have helped understand societal behavior, it was spoiled food or other sources of potential pathogens that elicited disgust. Shuar with the highest levels of disgust had the lowest load of infections.

Biomarkers of viral and bacterial infection in the blood and parasitic worm infection in the gut are generally associated with inflammatory responses, said Josh Snodgrass, a biological anthropologist in the Department of Anthropology. Linking pathogen-related data with disgust sensitivity and behavior, he said, was vital in understanding the relative costs and benefits of avoidance that are reflected in disgust responses.

Charles Darwin theorized that disgust evolved to avoid tainted food, the co-authors noted. Since the 1960s, Sugiyama said, numerous researchers from different fields have studied disgust, but not with fieldwork that captures its relationship to pathogen avoidance and how it changes in response to the local environment in real time.

All Shuar communities are exposed to fairly high pathogen loads, but they vary in their relationships to modern conveniences.

Some have palmwood houses, thatched roofs and dirt floors. Others have rough wood houses with tin roofs. Some obtain water from streams or rivers, while others are closer to roads and have varied access to spring-fed water systems, cook stoves, electricity, refrigeration, health care, sanitation and other outside products. Some subsist on traditional agricultural, fishing and hunting lifestyles, while others supplement how they live with wage labor or sales of agricultural products.

Individuals living in the most market-integrated households had higher levels of disgust sensitivity and lower levels of infection.

“We also found that household disgust levels correlated with community disgust levels,” Sugiyama said. “They share food; drinking bowls for their nijiamanch, a manioc beer that is a dietary staple; water sources; and exposure to soil and, thus, some pathogens. That shared disgust shown as a community helps to regulate the level of disgust in individuals.”

Disgust, he said, is relative to what a household and communities are doing in terms of market integration, associated with how hard or easy it is to exercise pathogen avoidance. Individuals regulate their disgust response across their lifetimes based on their immediate environment, Sugiyama said.

“This study provided a powerful analysis that let us appreciate the evolutionary perspective that our minds and bodies have been designed to help us deal with particular environments and threats like pathogens,” Snodgrass said. “By how we designed the study we were able to understand what our behavior looks like in terms of being safe and minimizing risks.”

Snodgrass and Sugiyama combine their various expertise in evolutionary psychology, human biology, evolutionary medicine, cultural anthropology, and lab and field methods to conduct research and train their doctoral students.

In addition to Cepon-Robins, the study was co-authored with five other former doctoral students: Aaron D. Blackwell, an associate professor at Washington State University; Theresa E. Gildner, an assistant professor at Washington University in St. Louis; Melissa A. Liebert, an assistant professor at Northern Arizona University; and Felicia C. Madimenos, an associate professor at Queens College, City University of New York.

Additional co-authors were former UO lab manager Geeta N. Eick and Samuel S. Urlacher, a former doctoral researcher at Harvard University now an assistant professor at Baylor University and a 2020-22 Azrieli Global Scholar in the Child and Brain Development Program of the Canadian Institute for Advanced Research.

The Wenner-Gren Foundation for Anthropological Research, National Science Foundation, National Institutes of Health through the Center for Evolutionary Psychology at the University of California, Santa Barbara, Ryoichi Sasakawa Young Leaders Fellowship Fund and Boettcher Foundation funded the research. Additional support was provided by the UO’s Institute of Cognitive and Decision Sciences, Department of Anthropology and faculty excellence and research awards.

—By Jim Barlow, University Communications