Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Disgusting Oral Origins of Moral Disgust


Science 27 February 2009: Vol. 323. no. 5918, pp. 1222 - 1226

In Bad Taste: Evidence for the Oral Origins of Moral Disgust

H. A. Chapman,1 D. A. Kim,1 J. M. Susskind,1 A. K. Anderson1,2

1 Department of Psychology, University of Toronto, Toronto, Ontario M5S 3G3, Canada.
2 Rotman Research Institute, Baycrest Centre for Geriatric Care, Toronto, Ontario M6A 2E1, Canada.


In common parlance, moral transgressions "leave a bad taste in the mouth." This metaphor implies a link between moral disgust and more primitive forms of disgust related to toxicity and disease, yet convincing evidence for this relationship is still lacking. We tested directly the primitive oral origins of moral disgust by searching for similarity in the facial motor activity evoked by gustatory distaste (elicited by unpleasant tastes), basic disgust (elicited by photographs of contaminants), and moral disgust (elicited by unfair treatment in an economic game). We found that all three states evoked activation of the levator labii muscle region of the face, characteristic of an oralnasal rejection response. These results suggest that immorality elicits the same disgust as disease vectors and bad tastes.

Although rationalist theories of moral psychology have long emphasized the role of conscious reasoning in morality, recent empirical and theoretical work suggests that emotion may also play a key role in moral judgment. These newer theories make the claim that moral cognition relies primarily on phylogenetically older affective systems, rather than on more recently evolved higher cognitive functions. For example, it has been proposed that the violation of moral norms might evoke a kind of moral revulsion or disgust in victims or onlookers. Disgust is a somewhat surprising candidate for a moral emotion, given its hypothesized origins in the very concrete, nonsocial, and straightforwardly adaptive functions of rejecting toxic or contaminated food and avoiding disease. In the moral domain, this rejection impulse might have been co-opted to promote withdrawal from transgressors, or even from the thought of committing a transgression. If the primitive motivational system of disgust is indeed activated by abstract moral transgressions, it would provide strong support for the idea that the human moral sense is built from evolutionarily ancient precursors.

The notion that moral transgressions might evoke the same disgust as potential toxins and disease agents has not gone unchallenged, however. Some have argued that just as a "thirst" for knowledge does not denote a desire to drink, moral "disgust" may reflect not the engagement of more primitive forms of disgust but merely the use of a compelling metaphor for socially offensive behavior. As well, prominent theories of disgust have proposed that although moral disgust may be related to contamination-based disgust (typically evoked by potential disease vectors), it is distinct from the most primitive forms of disgust related to the ingestion of potential toxins, having differentiated from the ancient oral distaste response rooted in chemical sensory rejection. Thus, the "bad taste" of moral disgust may serve as an abstract metaphor rather than reflect a concrete origin in oral distaste.

Our results provide direct evidence of the primitive oral origins of moral disgust. A facial motor action program evoked by aversive chemical sensory stimulation extends to other concrete forms of disgust related to cleanliness and contamination and is also triggered when the everyday moral code of fairness is violated. Furthermore, subjective feelings of tasting or smelling something bad were evoked in response to unfairness, and, in parallel with disgust-related facial motor activity, predicted increasing rejection of unfair offers. The disgust evoked by moral transgressions thus appears to be similar to that evoked by bad tastes and potential disease agents.

These results are consistent with the idea that in humans, the rejection impulse characteristic of distaste may have been co-opted and expanded to reject offensive stimuli in the social domain. Although some theories have proposed that moral disgust is reserved for transgressions that are conceptually related to notions of moral contamination or purity, with anger and contempt being the more likely response to violations of individual rights and community norms, our data suggest that moral disgust may in fact be triggered by a wider range of offenses. The role of disgust in active rejection and distancing could explain why immorality evokes this emotion in addition to others such as anger: Whereas anger is associated with approach motivation, disgust may motivate vigorous withdrawal. Thus, unfair offers may be received like a plate of spoiled food. This turning away or rejection of unfair actions may also extend to later avoidance of transgressors.

The ability to detect and avoid toxins appears to be very ancient: Sea anemones, which evolved about 500 million years ago, evert their gastrovascular cavities in response to being fed a bitter substance. 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. Although the stimulus triggers for this rejection mechanism may have shifted far from their chemical sensory origins to the moral domain, the basic behavioral program of oral rejection appears to have been conserved. Thus, the metaphorical "bad taste" left by moral transgressions may genuinely have its origins in oral distaste.

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