Moral disgust involves the same primitive emotional circuitry that makes us wrinkle our noses at the sight of a dirty toilet, Canadian researchers have found.
"Immorality is disgusting in the same way rotten food is disgusting," says Hanah Chapman, the University of Toronto doctoral student who is the lead author of a paper published in yesterday's edition of the journal Science.
Ms. Chapman works with Adam Anderson, an assistant professor of psychology at the U of T who is investigating the biology of human emotion.
They conducted experiments in which separate groups of volunteers were asked to drink something unpleasant, look at images of feces or a bloody wound or to play a game about dividing $10 in which they were treated unfairly.
The scientists attached two electrodes to each volunteer's face to measure the activity of the levator labii, the muscle that raises the upper lip and wrinkles the nose, a universal expression of disgust.
In all three experiments, that muscle was activated.
This suggests, Ms. Chapman says, feeling disgusted at someone's bad behaviour is similar to how we react to a bitter taste or a repugnant image.
Evolution, Ms. Chapman says, doesn't often invent something from scratch, but instead builds on what is already in place. Just as limbs grew from fins, the brain circuitry involved in moral repugnance is built on something more primitive.
It may date back 500 million years. Sea anemones, which have been around that long, turn their digestive tracts inside out when they taste something bitter, a response that protects them from toxins.
Dr. Anderson, who focuses on the primitive origins of complex emotions, found in an earlier experiment evidence to support Charles Darwin's theory that facial expressions evolved - and are the same in all cultures - because they serve a specific physical function. Take fear, for example. Our eyes widen and our nostrils flare when we are frightened, which enhances our ability to spot danger and take in more air in case we need to flee.
In disgust, the effect is the opposite. Our noses and eyes scrunch up, shrinking two entry points for the contagions that can be found in feces, vomit, spit, dead bodies and pus. Some researchers have argued that we find these things so disgusting because they can carry bacteria and viruses, and avoiding them helps keep us healthy.
But over time, facial expressions may have proven useful as social signals for humans, Dr. Anderson says.
Moral disgust doesn't have anything to do with avoiding hazardous food or illness, he says, but the fact that the facial expression is the same shows that it is born from the same emotion.
He has also looked at how the emotion of disgust affects memory, and found that people are better at remembering pictures of a dirty toilet or rotting food than images of snarling dogs or someone with a knife to their throat.
"Disgust, more than fear or anxiety, hijacks the attentional system," he says.
Perhaps, he says, one of the functions of social disgust is to help us to remember people who treated us badly in the past so we can avoid them.
It is hard to get over feelings of disgust. If you see someone put a spoon in feces, says Dr. Anderson, even if they sterilize it you don't want to eat off it. The same may be true of social disgust. This helps explains negative advertising, he says, and why some politicians seek to evoke a feeling of repugnance for their opponents.
