“Yuck!” Infection risks and parasites are known to cause a feeling in humans that is commonly referred to as disgust. However, depending on the trigger, the intuitive behavioral reactions differ in a meaningful way, a study documents. What appears to be microbially dangerous therefore creates stronger mechanisms for avoiding oral ingestion. Disgusting impressions from parasites, on the other hand, are more associated with reactions that can serve to protect the body surface. The results shed light on the complexity of the human “behavioral immune system” and can provide information on the basis of pathological reactions in some people, say the researchers.
Unpleasant to stressful: The feeling of disgust does not have a good image – but viewed objectively, it is a vital protective mechanism that, like fear, protects us from danger. The sensation is considered to be a mechanism that has developed in the course of human evolution as the basis for avoidance behavior to protect against pathogens and ectoparasites. Sensory impressions of spoiled food, excrement, body fluids as well as skin parasites that can potentially transmit diseases evoke complex disgust reactions and behaviors. But to what extent are they adapted to the two different trigger categories? The researchers headed by Tom Kupfer from Nottingham Trent University pursued this question as part of their study.
Disgust reactions recorded
They carried out investigations with a total of 1079 participants in the USA and China. The test persons saw a series of video clips that showed different scenes relevant to microbes or ectoparasites. On display were feces, recordings of skin diseases or depictions of ticks, fleas and the like. The test persons were asked to indicate on a form with various categories to what extent they perceived physical sensations when viewing a respective clip or what behavioral reactions they noticed in themselves. In addition to “nausea” or “itching”, this also included more detailed aspects such as “goose bumps”, “trembling shivers”, “oppressive feeling in the throat”, or the need for various forms of cleaning.
As the researchers report, it became apparent in their evaluations: Even if the study participants described their basic feeling with a general term for disgust, there were clear differences depending on the stimulus. Evidence of pathogens tends to trigger disgust reactions that are functionally linked to avoiding ingestible media that are potentially contaminated by microbes. The feeling in connection with the sight of ectoparasites, on the other hand, was comparatively strongly linked to sensory skin reactions and nursing behavioral needs, which can serve to protect the body surface.
In addition, it was found that signs of illness on human skin, such as warts or infected lesions, did not trigger the reactions typical of parasites, but rather oral-gastric effects. This indicates that a link with the skin does not generally lead to the effects. “The results show that humans have pronounced psychological and behavior-related defense mechanisms against ectoparasites that differ significantly from those when they react to pathogens,” the scientists sum up.
Nausea even with parasites
However, they also highlight similarities: “Despite the clear differences between the two classes of reactions, our results also show overlaps, which indicates an incomplete distinction between ectoparasite defense and pathogen defense mechanisms,” the scientists say. This is because the study participants also reported to a certain extent that they felt some oral-gastric sensations when they were stimulated by ectoparasites. As the researchers explain, the development of the psychological adaptations for health protection could have resulted in the use of a common basic neural architecture, which led to the overlap in the reaction patterns.
The study is thus now a contribution to understanding the complex “behavioral immune system” of humans, say the researchers. Specifically, the results and further studies could also provide clues to the causes of certain mental illnesses that are associated with feelings of disgust. For example, some forms of phobias or self-inflicted damage to the skin could be more associated with malfunctions in the ectoparasite defense mechanisms than with pathologies of microbe avoidance, write Kupfer and his colleagues.
Source: Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098 / rspb.2021.0376