Rotting food, uncovered sneezes and dirty bathrooms can all trigger a reaction so powerful it’s physical: wrinkling your nose in disgust.
It’s an instantly recognizable facial expression.
“Disgust is widely thought to be one of the basic emotions,” said Daniel Kelly, an associate philosophy professor at Purdue University who studies disgust. “It’s a piece of human psychology that is universal.”
That innate recoil, which can be triggered by off-putting physical cues or social behaviors, isn’t just an easily decipherable signal of our emotions to other humans. It may have also served an evolutionary function that helped our ancestors stay alive by avoiding potential threats.
It’s a two-part evolutionary story, Kelly explained. Humans are omnivores who consume a wide variety of foods, so an innate revulsion to rotting or foul-smelling substances protects us against potentially poisonous material that could make us sick or trigger gastrointestinal distress.
The second half of the disgust equation is like a front-line defense for the immune system. We might not consciously associate a dirty bathroom sink with disease risk, but humans have an instinct to avoid coming into close proximity with anything that might be contaminated with bacteria or an infectious disease, including fellow humans who might be sick.
Even seemingly innocuous, everyday sources of disgust, like clumps of hair in a bathroom sink or fingernail clippings scattered on a table, could have roots in an evolutionary impulse to avoid germs and bacteria. Hair on a person’s head could be seen as a sign of health and vitality, but a cast-off part of a person’s body, like a fallen hair or stray nail clipping, could potentially transmit disease.
“It’s not because you are going ‘Oh my goodness, I might catch an infection.’ It is because we have evolved to find the smell of poo disgusting so that we won’t go near it,” said Valerie Curtis, professor at the London School of Hygiene and Tropical Medicine and author of the book Don’t Look, Don’t Touch, Don’t Eat.
There are six broad categories of disgust that are “common everywhere in the world,” Curtis explained, noting that different cultures have slightly different variants on each type.
“There’s basically basic poor hygiene, food gone off, sex with people who are showing signs of sickness, animals and insects, lesions and wounds, and people of atypical appearance,” she said.
That’s where evolution can get dicey.
From an evolutionary perspective, if a person’s appearance is atypical in some way, it might set off alarm bells on a primitive level: Something may be wrong with this person; they might have a disease; best to avoid them. But in reality, just because there’s something unusual about a person’s appearance doesn’t mean they have a disease, or pose a risk of contamination. It’s an example of how our evolutionary impulses are at times out of step with modern disease risks and behavioral norms.
Discrimination based on appearance can have real-world consequences. Adults who have a disability, for example, are more than twice as likely to live in poverty as those who don’t.
It’s not just physical cues like rancid smells and rotting food that can trigger a wave of nausea or a wrinkled nose. Humans can have the same bodily responses to social behavior they find abhorrent ― something researchers refer to as moral disgust. (There is, however, some disagreement among experts about whether moral disgust is a “genuine” form of disgust or simply a metaphorical concept.)
“The emotion of disgust has been appropriated for other purposes, to make us stay away from people who are unhygienic and bad-mannered, but then also to make us stay away from people who are like social parasites, who actually threatened you with stealing your stuff or behaving badly or exploiting you,” Curtis said.
She pointed to the recent murder of journalist Jamal Khashoggi in Turkey. The grisly details of Khashoggi’s death in the press prompted a more visceral level of disgust among readers and viewers than simply talking about the immorality of murder might have.
“That’s what’s made the story so powerful,” Curtis said. “We’re making use of an ancient emotion in an extended way. You can harness disgust for political end.”
In some instances, like Khashoggi’s murder, our evolutionary disgust response tracks with an aversion to unjust killing, a generally shared moral viewpoint.
But in other cases, it might be better to rise above our evolutionary nature.
Kelly pointed out that evolution is not a “perfectly foresighted intelligent designer.” The human behaviors that trigger moral disgust are shaped by social norms and group membership, which are imperfect guidelines for distinguishing right from wrong.
Trying to invoke moral disgust against a whole group of people, for example, can fuel racism, homophobia or xenophobia, such as when President Donald Trump claimed that migrants entering the United States would “infest our country.”
The word “infest” is usually associated with insects and vermin, and could prompt a disgust response among people who hear Trump say it, thus dehumanizing the people and groups he targets and making them easier to disregard.
“We know that disgust has a tendency to be able to shape our moral views, but it’s not a fact of our nature that we should celebrate,” Kelly cautioned. “Just because you’re disgusted by something doesn’t give you a good reason to think that it is in fact morally wrong.”