- 19:00 26 February 2009 by Ewen Callaway
- For similar stories, visit the The Human Brain and Evolution Topic Guides
People’s visceral reaction to incest or betrayal by others could stem from a natural aversion to potentially toxic foods, researchers argue.
Subjects who swig nasty tasting liquids grimace in a similar way to people who view photos of open wounds or toilets covered in faeces, as well as people whose trust is violated.
This overlap points to a common neural foundation for distaste and moral disgust, say Hanah Chapman and Adam Anderson – both psychologists at the University of Toronto.
Common usage of the word disgust and its synonyms might reflect a deep connection between moral disgust and distaste, she says. “People will say that behaviour disgusts me, or so-and-so is repulsive, or that interaction left a bad taste in my mouth.”
Because people frequently employ the word disgust as a stand-in for anger, Chapman’s team wanted a more objective measure of the emotion. They 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.
Indeed, 27 volunteers in that study who sipped bitter, salty and sour liquids clenched their levator labii far more tightly than when they drank sweet liquids.
Another group of 19 volunteers contracted this muscle group while viewing disgusting images, but not at merely sad pictures – traffic accidents or homeless people, for instance.
These responses make sense, since it often pays to avoid bitter foods, as bitterness is an indicator of toxicity in plants, as well as bacteria-filled faeces, Chapman says. “Disgust is an avoidance mechanism at heart. It keeps you away from something that makes you sick.”
To connect these responses to more abstract forms of disgust, such as those evoked by thoughts of incest or the sight of Nazi uniforms, Chapman’s team tested another 21 volunteers on a task commonly used to gauge trust.
In a two-person “ultimatum” game, a researcher or computer makes an offer to split $10 in various ratios that test subjects can accept or reject. Unfair offers – $9 for the researcher and $1 for the volunteer – evoked significantly more levator labii activity than equitable offers, Chapman’s team found.
She speculates that moral revulsionevolved out of more primal forms of disgust to help people avoid untrustworthy individuals.
“We say things like ‘Bernie Madoff, he’s disgusting’, or ‘the stimulus package grosses me out’,” but these are all metaphors, says Paul Bloom, a psychologist at Yale University. He wonders if the subjects in playing the ultimatum game weren’t experiencing something along the lines of contempt, rather than true disgust.
Sense of contamination
Paul Rozin, a psychologist at the University of Pennsylvania, agrees that disgust may have piggybacked on the facial expressions generated by distaste, but says, “the fact that we use the word disgust and make a face doesn’t guarantee that you have that feeling”. Real disgust involves a mental calculation that distaste does not, he adds.
Chapman agrees that not all the qualities of disgust are reflected in taste aversion. Some people ignore bitter-tasting Brussels sprouts and push them to the side of their dinner plate.
That’s not the case with disgust, which also involves a sense of contamination, she says. “If I give you a scoop of poo on your plate you’re not going to eat your mashed potatoes next to them.”