Maggots. Rotten meat. Pus-oozing sores. Grossed out yet? Probably. The emotion of disgust is universal, strong and easy to invoke. A single disgusting photo is all it takes to make most of us say, "Ick."
And that's for a good reason. Just as fear protects us from a lion that would eat us, "disgust is quite similar. It keeps us away from tiny little animals that would eat us up from the inside," said Valerie Curtis of the London School of Hygiene and Tropical Medicine, the lead author of a paper published today in the Philosophical Transactions of the Royal Society . "We evolved to stay away from poo, from bodily fluids, from mucous, from foods that have gone off, from worms in the garden."
It's not just humans that have this reaction. Even nematode worms can recognize parasitic bacteria in a petri dish and crawl the other away, Curtis said. "It's a simple animal with only 302 neurons and it's got a disgust reaction."
While the emotion is universal, it is flexible -- we can learn to be disgusted by new things -- and its intensity varies from person to person and depending on the circumstances. Individual differences can be measured by tests of "disgust sensitivity," which scores how disgusted people are by typical gross things like feces or rotten meat.
Disgust may have evolved to protect us from pathogens, but it can go too far. Types of obsessive-compulsive disorders are thought to result from disgust sensitivity taken to the extreme, Curtis said, such as obsessive hand washing or boiling tea water multiple times before drinking.
More controversially, some argue that effects of disgust are even more far-reaching, influencing how societies operate and possibly forming the evolutionary foundations of morality.
"If I go around leaving poo in your front lawn or spitting in your cups or making nasty smells in public transport or if I go to church in my pajamas, I'm threatening you with my bodily fluids," Curtis said. "These are manners, but they're also the precursor of moral behavior. That's at least one of the ways that morality could have evolved in society: simple rules about not getting other people sick with your emanations."
Studies have even shown a connection between physical disgust and moral disgust, Curtis said. "If you sit people in a room with bad smells, they punish more severely," she said. "Your sense of disgust for people's bad behavior is tied together with your organic system."
Mark Schaller of the University of British Columbia in Vancouver has found that social behavior can change depending on people's fear of pathogens. "When the threat of disease is made salient to people, people become less sociable," he said.
"Societies with high pathogen risk tend to be societies that have a greater number of religions, they're more close knit, have more socially conservative rules and are more xenophobic," Curtis said, citing other work by Schaller and others.
"It might be that if you live in a society where you hear a lot about disease, your disgust sensitivity is going to be tuned up higher and as a result, you find that effect across society. It could also be that there are some group selection effects. If one village was really beset by some serious disease problems, would they tend to evolve towards higher disgust settings?"
Said David Pizarro of Cornell University, who researches moral judgment: "Even though I do think that the evolutionary approach is the right one to explain the origins of disgust and how it works in life now, I wonder if it can be applied too broadly."
"It seems unclear to me that you need an evolutionary approach to explain some of the behaviors [discussed]. For instance, avoiding large groups when you know that there's an outbreak of influenza. It seems people would just sort of notice. I don't know that it has to recruit a special system. There are a lot of things like manners that may or may not have anything to do with the avoidance of disease and seem sort of arbitrary."