Disgust’s evolutionary role

No longer. Disgust is having its moment in the light as researchers find that it does more than cause that sick feeling in the stomach. It protects human beings from disease and parasites,and affects almost every aspect of human relations,from romance to politics.

In several new books and a steady stream of research papers,scientists are exploring the evolution of disgust and its role in attitudes toward food,sexuality and other people.

Paul Rozin,a psychologist who is an emeritus professor at the University of Pennsylvania and a pioneer of modern disgust research,began researching it with a few collaborators in the 1980s,when disgust was far from the mainstream. “It was always the other emotion,” he said. “Now it’s hot.”

Speaking last week from a conference on disgust in Germany,Valerie Curtis,a self-described “disgustologist” from the London School of Public Hygiene and Tropical Medicine,described her favourite emotion as “incredibly important”. She continued: “It’s in our everyday life. It determines our hygiene behaviours. It determines how close we get to people. It determines who we’re going to kiss,who we’re going to mate with,who we’re going to sit next to. It determines the people that we shun,and that is something that we do a lot of.” It begins early,she said: “Kids in the playground accuse other kids of having cooties. And it works,and people feel shame when disgust is turned on them.”

It adds to the popularity of disgust as a subject of basic research that it is easier to elicit in an ethical manner than anger or fear. And disgust has been relatively easy to locate in the brain,where it frequents the insula,the amygdala and other regions. “It is becoming a model emotion,” said Jonathan Haidt of the University of Virginia,a disgust pioneer with Rozin.

The research may have practical benefits,including clues to obsessive compulsive disorder,some aspects of which—like excessive hand washing—look like disgust gone wild. Conversely,some researchers are trying to inspire more disgust at dirt and germs to promote hand washing and improve public health. Curtis is involved in efforts in Africa,India and England to explore what she calls “the power of trying to gross people out”. One slogan that appeared to be effective in England in getting people to wash their hands before leaving a bathroom was “Don’t bring the toilet with you.”

Disgust was not completely ignored in the past. Charles Darwin tackled the subject in The Expression of the Emotions in Man and Animals. He described the face of disgust,documented by Guillaume-Benjamin Duchenne in his classic study of facial expressions in 1862,as if one were expelling some horrible-tasting substance from the mouth.

It is generally accepted that disgust evolved partly to avoid putting bad things in the mouth,an idea already put forth when Rozin tackled disgust. He and his colleagues developed the idea that disgust was then elaborated by cultural evolution to include other forms,one of them based in a dislike for reminders of the animal nature of humans. Sex,death,faeces and bad food all smacked of animality.

The animal origins of disgust involve all sorts of ways that diseases are spread,including fleas,so there are a variety of signs of disease and types of disgust. Under that evolutionary umbrella,however,there is still the question of what kinds of disgust there are. Haidt,Rozin and Clark McCauley of Bryn Mawr College claim nine different domains of disgust for North Americans. Curtis proposes seven categories. Joshua Tybur of VU University in Amsterdam proposed three domains of disgust,three separate psychological programmes,for disease avoidance,mate choice and moral judgement. People who are sensitive to one type of disgust are not necessarily sensitive to another,” he said. For example,he said,earlier claims that political conservatives were more sensitive than liberals to disgust were overly general. Research that he and his colleagues did suggested that conservatives were more disgusted by sexual topics but were similar to liberals in the domains of disease avoidance and moral judgement.

Daniel Fessler,an anthropologist at the Center for Behavior,Evolution and Culture at the University of California,Los Angeles,investigated with his colleagues why pregnant women were more sensitive to disgust. What they found was that as progesterone levels went up,so did sensitivity to disgust. That was true in the first trimester of pregnancy,when derailing foetal development would have the most dire effects. In very recent work,Fessler said,the researchers found that even in women who weren’t pregnant and were not suffering nausea,disgust increased with the levels of progesterone.

An important function of progesterone,Fessler said,is that it dials down an early-warning part of the immune system,inflammation,which might prevent the embryo,or conceptus,from implanting itself in the placenta. The eight-cell embryo “actually destroys tissue as it burrows in,” Fessler said. “Left to its own,the maternal immune system would destroy the conceptus.” So,he and his colleagues reason,while the body turns down the dial on one kind of protection,it turns it up on disgust,another kind of defense.