The Yuck Factor and the Evolution of Moral Disgust

Conservatives often appeal to moral emotions such as disgust and anger as important for moral judgment. One example is conveyed in a phrase made famous by Leon Kass--"the wisdom of repugnance." If some proposed technique of biotechnology--such as cloning, for instance--elicits deep disgust in many of us, then, Kass suggests, this should be taken seriously as an expression of moral concern.

Kass's critics have rejected such thinking as irrational in allowing emotions to cloud our moral reasoning. After all, can't we easily think of many situations were feelings of disgust support immoral positions such as racism and anti-Semitism?

The defense of Kass's position has been hindered by the fact that Kass and his supporters have often failed to explain the philosophic and scientific tradition supporting the importance of moral emotions for moral judgments. As I have argued in a previous post, Kass's reasoning belongs to a tradition of moral naturalism that stretches from David Hume to Adam Smith to Charles Darwin--a tradition that recognizes the emotionality of moral experience and that denies that pure reason by itself can explain moral judgment.

Moral reasoning is important to moral judgment, because our moral emotions are responses to how we interpret the practical circumstances of life, and so we can reason ourselves into and out of our emotions depending on how we interpret those circumstances. As I have indicated in previous posts, the incest taboo illustrates this. Incest provokes deep disgust in the minds of most of us. But our individual reasoning and cultural learning influence what we consider to be incestuous behavior. Some people and societies regard cousin marriage as incest and thus disgusting. But those who conclude that cousin marriage poses very little risk of physical or mental disabilities in the offspring might feel no disgust with cousin marriage.

Now we have new research exploring the evolutionary and neurophysiological bases of moral disgust. The February 27th issue of Science has a research report by H. A. Chapman et al. entitled "In Bad Taste: Evidence for the Oral Origins of Moral Disgust" and a commentary by Paul Rozin et al. entitled "From Oral to Moral."

Here's the abstract for the Chapman article:

"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 oral-nasal rejection response. These results suggest that immorality elicits the same disgust as disease vectors and bad tastes."

The logic of this reasoning turns on the principle of preadaptation--what evolves for one purpose can be used for another purpose. What originated as a system for rejecting bad food could be extended--through genetic or cultural evolution--to a system for evaluating some things or thoughts as disgusting and thus causing withdrawal from those disgusting things or thoughts.

This research confirms Darwin's insights in his book The Expression of the Emotions in Man and Animals. In the Descent of Man, Darwin had argued for an evolutionary theory of morality as rooted in a natural moral sense expressed in moral emotions or sentiments. Here he followed in the tradition of Hume and Smith, who argued that without the motivational power of moral emotions, pure reason alone could not explain moral judgments. Moreover, Darwin tried to show how these moral emotions could have arisen from primitive animal instincts. His book on the emotions was designed to show that there were some universal facial expressions of human emotions that suggested primitive origins shared with other animals. Expressions of disgust were one example of a universal pattern suggesting moral emotions.

This new research correlating specific facial muscle movements with the disgust felt when human beings think about unpleasant tastes, cockroaches, incest, or unfairness (in the "ultimatum game") suggests the evolutionary origin of disgust in distaste.

This could explain why Hume and others in the Scottish moral tradition emphasized the importance of "good taste" in moral experience as grounded in human nature.

When I was a graduate student at the University of Chicago in the mid-1970s, I attended one of the first classes that Kass taught there. It was a class on the moral and biological psychology of the passions. The reading list included Darwin's Expression of the Emotions, as well as Book 2 of Aristotle's Rhetoric and Descartes' Treatise on the Passions. I was writing a dissertation on Aristotle's rhetorical theory, and I was interested in how Aristotle developed the moral psychology of the passions in the Rhetoric. I was also interested in how Darwin's work could be seen as uncovering the biological roots of such moral psychology in evolved human nature. It was clear that Kass had similar interests in the possibility of a Darwinian ground for Aristotelian moral naturalism. This was reflected in Kass's first book--Towards a More Natural Science--which influenced my thinking about "Darwinian natural right."

But then in later years, Kass began to move away from Aristotelian and Darwinian themes as he turned more and more to biblical writings, which culminated in his book on Genesis--The Beginning of Wisdom. He seemed to look more to an biblical ethics of divine command rather than an Aristotelian/Darwinian ethics of human nature. And yet his talk about the "wisdom of repugnance" shows a glimmer of that earlier attempt to explore the natural roots of morality in moral emotions.