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The news earlier this fall that chemical weapons had been used in Syria’s civil was seen as a new low in that conflict. Many people condemned their use as “disgusting”; President Obama, making the case for a military response, said the images from the attack were “sickening.”

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Moral outrage sometimes demands the language of disgust. In the case of serious and grave wrongdoing, it feels appropriate that physical revulsion overlaps with moral repulsion.

Are physical and moral disgust truly connected in our minds, or do we simply borrow the rhetoric of disgust to show strong disapproval?

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Valerie Curtis, director of the Hygiene Centre at the London School of Hygiene and Tropical Medicine and author of Don’t Look, Don’t Touch, Don’t Eat: The Science of Revulsion, believes that disease avoidance is at the core of both physical and moral disgust. Although there is some cultural variability in what we find disgusting, people all over the world tend to be disgusted by similar things. Vomit, feces, and putrid flesh are triggers for disgust in every culture, and each is an obvious source of infection. Curtis says that “disgust is a system in your brain that biases your behavior to stop you coming into contact with things that might make you sick,” and this includes both unhygienic people and substances.

According to Curtis, moral disgust evolved out of physical disgust and originally served the same purpose of infection avoidance. We need to interact with others, but we don’t want to risk contamination with their bodily fluids. So we work out a system of manners and rules so that we can interact without contamination. This is the beginning of morality, says Curtis. We are disgusted by social “parasites” just as we are by sources of (literal) disease and infection. Our disgust at immoral behavior makes us shun perpetrators, who in turn become ashamed and less likely to break moral rules in the future.

Psychologist Paul Rozin has defended a different view. According to him, disgust is deeply connected with the ideas of contagion and contamination rather than with genuine sources of disease. For example, Rozin and his colleagues found that adults would not eat chocolate that had been formed into the shape of dog feces. The disgust aroused by poop was transferred onto the harmless (and delicious) chocolate. According to Rozin, the things that disgust us are not only those that might make us sick, but also those things that remind us of our animal nature. Humans eat, excrete, and copulate like other animals. Yet we are deeply uncomfortable with the idea that we are, in fact, animals, and like all other animals, we’re destined to die. Disgust functions as a “defense mechanism” to keep awareness of our animal nature and our mortality at bay. Once disgust is in place as a “guardian” of the physical body, it can be elicited not only by things that could make us ill, but by very different kinds of stimuli. Each human culture “co-opts” disgust, says Rozin, and projects the emotion onto people and behavior it considers immoral, even if they present no significant risk of disease.

When we look to the evidence for deeper connections between physical and moral revulsion, we find a mixed picture. Certainly crimes of violence and attacks on the innocent can make our stomach churn. There is evidence that people have stronger and more typical disgust reactions to moral transgressions that involve bodily fluids. Yet disgust seems not to be associated with moral wrongs that turn on injustice, such as property crimes and financial misdeeds. Are these, even if they result in harm, genuinely disgusting or is calling them that simply a way to voice our anger and disapproval?

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Researchers at the University of Toronto thought that they had found evidence that people who have been cheated against in a game make the same kind of characteristic “disgust” facial expression as those who have tasted a bitter drink or looked at a disgusting image. That may seem to support Rozin’s idea of disgust—that the idea of injustice itself is repugnant. But critics pointed out that the raising of the upper lip—which the Toronto researchers took as the principal measure of disgust—is also activated by anger. So it could be that the experimental subjects were angry rather than disgusted, or perhaps some mixture of the two.

Curtis calls moral disgust a “double-edged” sword. When directed against “little infractions, little discourtesies,” it helps support the fabric of society. Yet there is a long, ignoble tradition of portraying social outsiders and outcasts as disgusting and as sources of physical or social ills, all the better to marginalize and persecute them.  Moral disgust has all too often been put to immoral ends.

Jeanette Bicknell, Ph. D., is the author of Why Music Moves Us (2009). She lives in Toronto, Canada.

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