Types of revulsion linked to the evolution of disease avoidance. Jeff Glorfeld reports.
It’s a common refrain, heard in a wide variety of contexts and situations. But now researchers have established that the emotion of disgust is likely an evolutionary trait structured around the people, practices and objects that pose disease risk.
Their report, published in the journal Royal Society Philosophical Transactions B, is the first time researchers have used the perspective of disease to break down the emotion of disgust into its component parts. It identifies six common categories that acts as triggers:
1. Poor hygiene, especially physical evidence of unhygienic behaviour.
2. Animals and insects, such as mice and mosquitoes, that represent disease vectors.
3. Behaviour denoting promiscuous sexual activities.
4. Atypical appearance, such as infection cues in people, including abnormal body shape, deformity, behaviour such as wheezing or coughing, and contextual cues related to high infection risk, for example homelessness.
5. Lesions, and other stimuli related to signs of infection on the body, including blisters, boils or pus.
6. Food items that show signs of spoilage.
London-based researchers Val Curtis, from the Department of Disease Control at the London School of Hygiene and Tropical Medicine, and Mícheál de Barra, from the Centre for Culture and Evolution at Brunel University, say their findings could help to target public health messaging – for example, to encourage handwashing with soap or to counter the stigma associated with sickness.
They say their findings confirm the “parasite avoidance theory”, in which disgust evolved in animals, thus prompting behaviours to reduce the risk of infection. Similar behaviours are replicated in humans, such that disgust signals us to act in specific ways that minimise the risk of catching diseases.
“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,” Curtis says.
"This type of disease avoidance behaviour is increasingly evident in animals, and so leads us to believe it is evolutionarily very ancient.”
The report says this explanation has not always been accepted, noting Freudian models in which disgust served as a defence mechanism against inappropriate desires, including incest. Charles Darwin described it as a feeling related to the avoidance of offensive foods.
The study surveyed more than 2500 people online, mainly from North America and Britain, listing 75 potentially “disgusting” scenarios they might encounter, ranging from people with obvious signs of infection, pus-filled skin lesions and objects teeming with insects, to listening to sneezes and defecation in the open. Participants were asked to rate the strength of their response to each scenario on a scale ranging from “no disgust” to “extreme disgust”.
Infected wounds producing pus were rated as the most disgusting. The violation of hygiene norms – such as having bad body odour – was also found to be particularly disgusting.
The six common categories each relate to regularly occurring types of infectious disease threat in our ancestral past. Historically, for example, eating rotting food could lead to diseases such as cholera, and close contact with unhygienic people could have transmitted leprosy. Promiscuous sexual practices might put an individual at risk of syphilis, and contact with open wounds could have led to plague or smallpox infection.
Curtis says increased understanding of disgust could provide new insights into the mechanisms of disease avoidance behaviour, and help us develop new methods of promoting healthy environments.
In analysing the survey results, Curtis and de Barra noticed gender differences in reactions to the disgusting scenarios that were presented. Women responded more strongly than men to every category.
They say their six-factor categorisation is likely to reflect a pathogen detection system that could not see microscopic pathogenic microbes and parasites directly, but could only evolve behavioural responses to categories of perceptible cues as to what to avoid: the people, practices and objects that have tended to occur with infectious disease.
They say the study provides further evidence that disgust serves to prevent infectious disease, among other functions, and that corresponds to an evolutionary view of emotions that trigger action. In general, emotions make us do things that put us in a better state with respect to our survival and reproduction.
“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,” de Barra says. “Our long coevolution with disease has wired in this intuitive sense of what can cause infection.”