Moral Disgust May Have Evolved From the Response to Rotten Food

By Eliza Strickland | February 27, 2009 8:50 am

disgusted expressionBeing treated unfairly in a game triggers the same facial expression as stomach-turning tastes and images, a new study has found, suggesting that the brain mechanism of disgust evolved to help humans avoid not just rotten food, but also immoral behavior.

“Our idea is that morality builds upon an old mental reflex, said study co-author Adam Anderson…. “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” [Wired News].

In the study, researchers had volunteers taste beverages that ranged from sweet to neutral to horribly bitter, and showed them a range of images including sad scenes (of, for example, car crashes) and disgusting scenes (of filthy toilets and the like). They also had the volunteers play a game with a partner in which the partner decided how to split $10 between them, and the volunteer had to decide whether to accept the terms. The results, published in Science, showed that bad tastes, revolting images, and blatantly unfair offers in the game all provoked the same response.

Study coauthor Hanah Chapman explains that the researchers relied on electrical measurements of a facial muscle group called the levator labii, which runs along our cheeks. It wrinkles the nose and purses the lip. A previous study from Chapman’s colleagues found that the levator labii muscles flex in response to disgust, but not anger. “It’s really quite specific,” she says [New Scientist].

The scrunched-up expression is a sensible response to a bad smell or taste, as it reduces odors and particles coming in through the nose and mouth. When new forms of social disgust evolved, that facial expressions may have simply tagged along with the brain mechanism. Chapman speculates that moral revulsion evolved out of more primal forms of disgust to help people avoid untrustworthy individuals…. “Disgust is an avoidance mechanism at heart. It keeps you away from something that makes you sick” [New Scientist], she says.

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Image: Science/AAAS

CATEGORIZED UNDER: Human Origins, Mind & Brain
  • Bongernet

    Morals evolved along with us.


  • Randy T

    I can see a strong linkage between distaste and disgust…body odors and the touching of the mouth to another person. This wouldn’t happen if moral disgust were a factor.

  • Stephen Daugherty

    Could it be (and I’m not trying to be freudian here) that children learn to associate disgust and disapproval together through their parent’s paired disgust and disapproval of their bodily functions, and that natural selection encouraged this pairing further because of its side benefits?

  • westernchuck

    “Moral Disgust May Have Evolved From the Response to Rotten Food”

    geez….the stupid burns….where do these morons get this stuff?

  • Joseph Ting

    Read: The surprising moral force of disgust. Boston Globe. Drake Bennett. 15th Aug 2010.

    The underpinnings of the moral imperative, be it emotional reactions based on inherent disgust or post-hoc moral justification, do not always work in synergy. They are at cross-purposes in certain situations-that murder is widely viewed as inherently reprehensible or disgusting could be relatively outweighed by the rationalization of a greater threat to the community or personal interests if the intended victim is not killed. The role of the “moral majority” in the institionalization or legalization of a moral view point adds further complexity to an often pliable moral environment. In France and Switzerland, women who wear Islamic face veils in public have recently been perceived to be far more of a threat than non-normative sexual behaviour. This situation is reversed in the Islamic world, where being gay is immoral whilst the face veil is considered a virtue. Such diametrically opposed attitudes reflect a moral flexibility based on society’s tolerance of difference, the degree of deviation or conformity with what is practiced by the majority and the (in) visibility of non-normative behaviour. The only non-contentious moral imperative could be that a behavior should not be found morally objectionable if it hurts no one, including oneself, in any material way.


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