Immorality is literally disgusting: it appears to provoke an ancient brain system designed to identify rotten food.

Disgust was observed in test subjects who, given an unfair offer in a money-splitting game, literally turned up their noses. The response was the same as to foul-tasting drinks and disgusting pictures.

"Our idea is that morality builds upon an old mental reflex," said study co-author Adam Anderson, a University of Toronto psychologist. "The brain had already discovered a system for rejecting things that are bad for it. Then it co-opted this and attached it to conditions much removed from something tasting or smelling bad."

Though philosophers have traditionally considered morality to be the pinnacle of purely rational thought, scientists have wondered whether emotion might also play a role.

Foremost among these emotions is disgust. The latest findings, published Thursday in Science, suggest a key role for the neurobiology of disgust, though Anderson notes that it's not the entirety of morality.

"You reject food that's unsafe to eat," said Anderson. "In moral cognition, you reject an action. We can't point to what comes first, but I don't think it's disgust. They're wrapped together, and disgust influences the decision being made."

Anderson's team recorded the facial reactions of people as they drank sweet, tasteless and bitter liquids, then looked at sad, neutral and disgusting photographs. From their reactions the researchers created a composite image; disgust clearly produced "activation of the levator labii region" — in layman's terms, a scrunched nose and raised upper lip.

Test subjects then played a game in which two people split 10 dollars, with the first player deciding the split and the second given the choice of taking or leaving it. Presented with unbalanced allotments — nine dollars for the first player, one dollar for the second — most test subjects called the offers unfair, said they felt disgusted, and rejected the money with an expression of distaste on their face.

In evolutionary terms, levator labii activation makes sense: foul odors or stray food particles are blocked from entering the nose and mouth. It also makes sense, said Anderson, for this tendency —

rather than, say, the confusion of fear, or the blind aggression of anger — to be adapted for guiding responses to unacceptable social situations.

In a critique accompanying the article, University of Pennsylvania psychologists Paul Rozin and Jonathan Haidt, along with University of

Virginia psychologist Katrina Fincher, were unconvinced by the results.

"It's unclear whether it's 'the same' disgust" aroused by immorality as by rotten food, they write, "or just common elements in the output system." Reactions to moral situations, they say, need to be tested before the role of disgust is clear.

But to Anderson, if people look disgusted and say they feel disgusted, then they're disgusted — and even if they didn't feel exactly the same about lowball offers as nauseating drinks, there was plenty of overlap.

"It's less important that the reactions are absolutely similar than generally similar," he said.

Disgusted test subjects reported feeling other emotions, including anger, fear and sadness, though disgust was most pronounced. To see if other types of cultural socialization might produce moral reactions with different physiological guides, Anderson plans to conduct the experiment with people from non-English speaking cultures.

"In a more collectivist culture, where part of being a person is being enmeshed in this larger cultural framework, maybe people would feel sadness as someone cheats them," he said.

Asked whether the findings suggest that moral judgments are dictated by physiology, Anderson stressed that morality is far more complicated.

"It involves a lot of thinking. Our results show that part of our moral compass is being guided by these older brain structures — but that's not the whole story," he said. "This system doesn't make decisions of fairness and unfairness. That happens in a higher-order part of the brain that feeds down to the primitive system, which feeds back up to the thinking centers. Emotion and cognition are intertwined."

Neither do the findings suggest a free pass for wrongdoers who would dismiss morality as a side effect of evolutionary jury-rigging. After all, babies are capable of disgust, but can't make adult judgments.

"Moral judgments involve something that's cultivated," said Anderson. "When you start talking about the brain, it doesn't release people from culpability or responsibility."

Citation: "In Bad Taste: Evidence for the Oral Origins of Moral Disgust." By H. A. Chapman, D. A. Kim, J. M. Susskind,

A. K. Anderson. Science, Vol. 323 Iss. 5918, Feb. 26, 2009.

"From Oral to Moral." By Paul Rozin, Jonathan Haidt, and Katrina Fincher. Science, Vol. 323 Iss. 5918, Feb. 26, 2009.

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