"I'm pretty inured to disgust," Curtis, who is the author of the book "Don't Look, Don't Touch, Don't Eat" (Chicago University Press, 2013), admitted to Seeker. "But like most people, I find excreta and bodily emanations pretty revolting."
Feelings of disgust turn out to facilitate infectious disease avoidance, according to new research conducted by Curtis and co-author Mícheál de Barra of Brunel University London. Their study, published in the Royal Society journal Philosophical Transactions B, strongly supports a theory holding that disgust functions to reduce contact with pathogens and parasites.
The scientists began their investigation by selecting a number of infectious diseases at random from a commonly referenced handbook concerning communicable disease epidemiology and control. Among those diseases, they extracted information about forms of transmission, such as via direct skin contact or airborne germs.
The researchers then generated a series of 75 scenarios that could, to varying degrees, mirror actual cues for those modes of disease transition. For example, exposure to bodily waste and other excretions was one of the identified infectious disease transmission routes. Their scenarios then included everything from seeing un-flushed excrement in a toilet to sitting next to someone on an airplane who is vomiting into a paper bag.
They also included some scenarios that might not seem so obviously off-putting. Since animals can be disease vectors, scenarios included imagining holding a "fat wriggling worm in your bare hands for 60 seconds," having a stray dog lick your face, and feeling a "hairless old cat" rubbing against your leg.
RELATED: Babies Appear to Understand Human Emotions at a Very Early Age
Still others concerned having "a woman with unkempt hair and disheveled clothes" sit beside you on a bus, "seeing an obese woman sunbathe" and "shaking hands with someone missing a thumb."
All of these scenarios were loaded into an online survey that was fully completed by 2,679 participants who were mostly from the US, UK, and Canada. For each survey entry, the participants were required to rate it on a spectrum with "no disgust" at one end and "extreme disgust" on the other.
The ratings showed that the scenarios most associated with sensorial cues for disease transmission routes were more often rated as being disgusting. The researchers further identified six basic types of disgust, based on the survey responses. They are disgust related to: hygiene, animals, sex, atypical appearances, lesions, and food.
Women were more likely than men to rate scenarios tied to all of these types as being disgusting. Women were also more often rated sex-related scenarios, such as seeing blisters or red dots on a partner's genitals, as being disgusting.
The survey respondents had to consciously consider each scenario, but if they were to have experienced these imagined instances in real life, their reactions would have occurred at both subconscious and conscious levels, Curtis suspects.
"All behavior is, to some extent, genetically programmed," she explained. "Our genetic makeup makes us enjoy the taste of sweetness and reject bitterness — somewhat modulated in later life by experience — for example. So, the sight or smell of feces produces a reaction that may happen before we are even conscious of it."
Disgust and fear can be interconnected, since both are adaptive systems that have evolved to enable avoidance of potential dangers.
"The point of these emotions is that they drive behavior, hence there is a physical and behavioral reaction," Curtis said. "There is also often a bodily reaction: The stomach churns in the expectation that it might need to eject something; the skin ‘crawls’ in expectation that one might need to scratch off a flea, for example."
Fleas themselves likely have their own evolved hygiene standards. Prior research has found that numerous animals behave in ways that may reduce their contact with harmful agents. Lobsters and mice, for example, avoid infected others. Nematodes and kangaroos steer clear of waste material. Birds and ants exhibit multiple hygienic behaviors.
Curtis, however, thinks it is unlikely that many non-human animals experience disgust as people do. She suspects that most of their disgust-related responses happen at a subconscious level.
A problem for humans is that our hardwiring for disease transmission cues may not always match real threats. Shaking hands with a person missing a thumb and viewing an obese individual sunbathing present no danger. Nevertheless, some people link such moments to feelings of disgust.
"In our evolutionary past, someone obese might be swollen up because of a disease, like filariasis," Curtis said. Filariasis is a parasitic disease caused by an infection with certain roundworms.
Nowadays, few people are overweight because of infectious illnesses, yet the past association may somehow be part of our genetic programming. "Disheveled" people, in turn, may be physically healthy, yet could elicit negative reactions for similar reasons.
RELATED: Storytelling Promoted Egalitarian Values Before the Advent of Religion
Humans may also view some perfectly fit animals as disgusting, simply because of similar long-held associations. A worm, for instance, "is a cue, looking a lot like a parasitic worm," Curtis said.
These mismatches between feelings of disgust and actual threats grow ever more complex when moral disgust is considered. The researchers did not tackle that loaded topic but theorize that moral disgust may have arisen as an extension of "hygiene disgust." Both forms can be affected by cultural factors.
The disgust system in humans is clearly imperfect, but it is at least useful to scientists in all of its respects.
"We need to understand how people respond to disease threats so that we can design programs to help people behave in ways that keep them safe from disease," Curtis said.
"So, for example, in another prior study where we wired up wash basins in public toilets in the UK, we found that people responded to messages such as ‘don’t take the toilet with you’ by washing their hands with soap more often," she continued. "In India the government very successfully uses disgust-based messaging to get people to build toilets."
She said that the disgust system also provides a great model for investigating other emotions with particular behavioral functions. Curtis shared that she and de Barra are now conducting these additional studies using similar methods in order to "pick these emotions apart."
A key to that ongoing effort, she said, is to consider what frequent related challenges early humans encountered.
"Evolution actually selects brains based on the behavior that helped our ancestors to survive and reproduce,” she said. Emotions are for behavior."