Chapman and her team recorded the facial muscles of participants when they tasted unpleasant liquids, when they looked at gory pictures, and when they were conned in a financial game. Throughout, the same muscles controlling the wrinkling of the nose and raising of the upper lip were activated.
What's more, when they were conned in the game, the participants tended to report that a picture of a disgusted facial expression, as opposed to other emotional expressions, best captured how they were feeling (anger and sadness were also sometimes reported, but to a lesser degree).
Finally, the participants' experience of disgust appeared to influence their behaviour. The more the participants' wrinkled their noses and curled their lips, and the more they said the picture of a disgusted face reflected their feelings, the more likely they were to reject an unfair offer in the financial game. By contrast, levels of self-reported anger and sadness were not linked with rejection decisions or nose wrinkling.
Taken together these findings support the idea that our moral sense has co-opted an evolutionarily older brain system - the Yuk! response - that serves to protect us from unpleasant, potentially toxic foods and substances. This implies that unfairness leaving a bad taste in the mouth is more than a mere metaphor, and is consistent with previous, related research showing, for example, that washing can assuage guilt.
"That a system with the ancient and critical adaptive function of rejecting toxic foods should be brought to bear in the moral sphere speaks to the vital importance of regulating social behavior for human beings," the researchers concluded.
Writing a commentary on this research in the same journal issue, Paul Rozin and colleagues cautioned that the detail is rather more complicated than the Chapman study implies. "Until studies examine the effects of a variety of elicitors on a variety of dependent measures (e.g., contamination, appraisals, and feelings)," they argued, "it is unclear whether [fairness and toxins provoke] 'the same' disgust, or just some common elements in the output system."
Link to related Digest items.
Image copyright: Science/AAAS
H. A. Chapman, D. A. Kim, J. M. Susskind, A. K. Anderson (2009). In Bad Taste: Evidence for the Oral Origins of Moral Disgust. Science. In Press.
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