YUCK, gross! But what kind of gross, exactly? The emotion we call “disgust” actually encompasses several different kinds of revulsion, each with their own evolutionary history and pattern of brain activity.
Most animals with the ability to select their foods are likely to have evolved something like disgust toward putrid morsels, so minimising their contact with pathogens. But with our relatively sophisticated social behaviour, humans may have co-opted this disgust mechanism to avoid sexual encounters with unsuitable mates such as siblings, and later added a third form of disgust directed at moral transgressions such as theft.
“Humans may have co-opted the disgust mechanism to avoid having sex with unsuitable mates, such as siblings”
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To test this idea, Debra Lieberman at the University of Miami, Florida, and her colleagues asked 50 young men to imagine one of four types of activity with their sister. Three of the scenarios were designed to trigger different kinds of disgust: pathogen-related disgust (for example, sipping their sister’s urine); sexual disgust (sex with their sister); and moral disgust (stealing her purse). A fourth scenario was a neutral activity such as sitting in the park with their sister. Meanwhile, the team monitored the volunteers’ brain activity with an fMRI scanner. The three types of disgusting stimuli triggered different but overlapping patterns of brain activity, the researchers found. They concluded that a single emotion – disgust – is applied in different ways to the three classes of stimuli ().
The three-way division of disgust gives a clearer insight into what disgusts whom, the team claims. For example, women tended to score much higher on measures of sexual disgust. This makes evolutionary sense, as women must carry a fetus during pregnancy and so have much more to lose from an ill-advised sexual encounter. “The sex difference in sexual disgust is huge compared to the other domains,” Lieberman notes.
Her study is valuable evidence that disgust spreads over multiple domains, says Daniel Fessler at the University of California at Los Angeles. However, he says that the study cannot cleanly separate sexual from moral disgust because incest has elements of both. Replacing incest with a less morally charged stimulus – by having the young men imagine sex with an 80-year-old neighbour, for example – might give a better test, he says.
The Human Brain – With one hundred billion nerve cells, the complexity is mind-boggling. Learn more in our cutting edge special report.
