Nobody likes being disgusted, but while this overwhelming emotional response may make you feel sick to your stomach, it turns out that's a good thing.
That's because scientists think intense feelings of disgust and revulsion are an evolutionary response to protect us from pathogens and other kinds of infectious threats – and now researchers have identified the six most common 'domains' of disgust, which, as bizarre as it sounds, are actually good for you.
"This type of disease avoidance behaviour is increasingly evident in animals, and so leads us to believe it is evolutionarily very ancient," explains disease control researcher and 'disgustologist' Val Curtis from the London School of Hygiene & Tropical Medicine.
"Although we knew the emotion of disgust was good for us, here we've been able to build on that, showing that disgust is structured, recognising and responding to infection threats to protect us."
To break down these archetypal forms of disgust from a disease avoidance perspective, the researchers surveyed more than 2,500 people, presenting them with a gallery of grossness: 75 revolting scenarios potentially suggestive of infectious vectors, which the participants had to rate on a scale from 'no disgust' to 'extreme disgust'.
Nothing was off limits. Things like: accidentally borrowing someone else's roll-on deodorant; noticing small red dots on your lover's genitals; or feeling someone cough into your face.
Participants also had to rate the prospect of being licked by a stray dog; sharing an office with a co-worker who has a weeping eye infection; squashing a slug in bare feet; and sitting in front of a vomiting man on an airplane.
By analysing the responses, the team was able to identify six common categories of disgust related to infectious disease:
• Hygiene: displays of, or physical evidence of, unhygienic behaviour.
• Animals/insects: such as mice and mosquitoes that represent disease vectors.
• Sex: behaviour pertaining to promiscuous sexual activities.
• Atypical appearance: infection cues in other people, including abnormal body shape, deformity, behaviour such as wheezing or coughing, and contextual cues related to high risk such as homelessness.
• Lesions: stimuli related to signs of infection on the body surface such as blisters, boils, or pus.
• Food: food items that show signs of spoilage.
While the researchers expected the results to correspond directly to different types of disease threats, they do so only to the extent that people can pick up on infectious vectors from obvious cues in the biological and social environment around us.
People are people, after all, not microscopes.
"It appears that cues to infectious disease threats are not categorised following the abstract biomedical categories of disease transmission risk recognised in the literature … but, rather, as categories of recognisable cues as to what to avoid," the researchers explain in their paper (original emphasis).
"These include potentially contaminated objects such as bodily fluids, infected lesions, spoilt foodstuffs, and animals that vector disease, practices, such as those that run the risk of contracting sexually transmitted diseases, and people who display visible signs of disease or poor hygiene."
Interestingly, the results showed that women rated all six categories of disease stimuli more disgusting than male participants, with risky sexual behaviour and animals carrying disease eliciting the most disgust from women in particular.
The researchers also speculate analogues of these six disgust categories may be found in other species – as it's not only humans who have had to evolve this kind of "pathogen detection system": or, in other words, things in our environment we know better than to touch.
"Although we only really came to understand how diseases transmit in the 19th century, it's clear from these results that people have an intuitive sense of what to avoid in their environment," says one of the team, evolutionary psychologist Mícheál de Barra from Brunel University London.
"Our long coevolution with disease has 'wired in' this intuitive sense of what can cause infection."
The findings are reported in Philosophical Transactions of the Royal Society B: Biological Sciences.