Among Ecuador's Shuar, researchers find how disgust evolved as a human emotion

In a project that blended anthropology, biology and psychology, UO researchers explored disgust behaviors among Ecuador's indigenous Shuar people. Those living in the most market-integrated households were found to have the highest levels of disgust sensitivity.

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 co-author Lawrence Sugiyama, a professor in the UO's Department of Anthropology.

"It's a behavior," he said, "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 numerous research projects done in almost 30 years of fieldwork by Sugiyama among indigenous forager-farming populations in Ecuador's Amazonian region.

In her project, Cepon-Robins focused on 75 individuals, ages 5-59, in 28 households in three Shuar communities that varied in their isolation from or partially involvement in the larger market economy. Her team explored measurements of disgust sensitivity to sources of infection, as well as the levels of bacterial, viral and parasitic infections identified from blood and fecal samples collected as part of 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.

Biomarkers of viral and bacterial infection in the blood and parasitic worm infection in the gut generally are associated with inflammatory responses, said co-author 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 had theorized that disgust evolved to avoid tainted food, the co-authors noted in their study. 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.

Shuar communities are exposed to fairly high pathogen loads, but they vary in their relationships to modern conveniences. Some subsist on traditional agricultural, fishing and hunting lifestyles, while others supplement how they live with wage labor or sales of agricultural products.

Living conditions vary. Some communities 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, healthcare, sanitation and other outside products.

The researchers found that individuals 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.