History of disgust

In contrast to the deep roots of pain research, the interest in disgust started only at the early modern period. The very first definition of disgust was made in 1872, by Darwin, as "something revolting, primarily about the sense of taste, as perceived or vividly imagine, and secondarily to anything which causes a similar feeling, through the sense of smell, touch and even of eyesight"¹³³. This definition already recognized the enormous variety of elicitors that can induce the feeling of disgust. The second

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landmark occurred in 1941, when the psychoanalyst Andras Aigle defined disgust as “a revulsion at the prospect of oral incorporation of an offensive object”, and identified body waste products as a focus of disgust¹³⁴. Since Aigle’s definition, different views have emerged considering disgust as an emotion¹³⁵, an aversion that centers on oral rejection¹³⁶, a bad feeling about another person's action¹³⁷, and a defense response against infection¹³⁸.

Another difference from pain research is the fact that most renowned theories agree at classifying disgust as an emotion. This classification was mainly shaped by early theories of emotion (around 1960-1970)102,138–143. For instance, disgust has been included as one of the “basic emotions” in Ekman’s model^139–142, according to which (together with anger, fear, enjoyment, sadness and surprise) it is characterized by specific behavioral, physiological – peripheral or neural – and subjective responses^143,144.

Despite the consensus on viewing disgust as an emotion, there is still a widespread disagreement over how to classify types of disgust into psychologically or functionally meaningful categories. As disgust can be triggered by many elicitors (e.g. bad tastes, bad smells, disgusting images, contact with strangers, moral offenses - see^145,146 for an overview), recent theories have proposed different assortments of these elicitos into distinct categories, by relying on common features and evolution-based evidence.

The most influential taxonomy was developed in 1987 by Rozin and Fellon¹⁴⁷ (and later was extended by Rozin and Haidt^145,148), which proposed a course of biological and cultural evolution of disgust, and is the basis for scales and tasks measuring individual differences in disgust sensitivity. Accordingly, disgust evolved from a food-related emotion that protects against poison (core disgust) and contamination (animal nature disgust), to a much more general emotion that protects from bodily (animal nature disgust) and social (interpersonal and moral disgust) harms. This evolution has been proposed to occur in several stages, starting from an ancient precursor of disgust that can be found both in animals and humans. Specifically, it is believed that the origin of disgust is distaste, a rejection response to bad tasting (bad sensory properties) foods, prototypically those that are bitter¹⁴⁵. The oral rejection is mostly achieved by spitting out the unpleasant substance. This rejection was observed in adult humans¹⁴⁹, and neonates only a few hours old^150,151, as well as non-human animals, including rats¹⁵², apes and monkeys¹⁵³. Because many toxins are bitter, it is reasonable to assume that the evolutionary function of distaste is in motivating the avoidance of poisonous foods¹⁵⁴. Indeed, the ability to detect and reject bitter substances seems to be very ancient: even sea anemones, which first evolved nearly 500 million years ago, will reject bitter foods from their gastrovascular cavity¹⁵⁴. Moreover, it was also suggested that the relationship between bitterness and toxicity might represent

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a form of coevolved communication between predators and prey¹⁵⁴. As the bitter taste associated with toxins warns potential predators about the cost of consuming a particular prey, both sides benefit from this communication. The predator avoids ingesting the toxic prey, and the prey avoids extensive injury.

As evolved from distaste, core disgust is considered to be the original form of disgust, which is involved in defending against infection and contaminations via the oral route¹⁴⁵. Three elements are compulsory to elicit core disgust: a sense of potential oral incorporation, a sense of offensiveness, and contamination potency¹⁴⁷. As the purpose of core disgust is to protect the mouth from diseases and infections, its elicitors can be identified as food and food-related contaminants (body products and some animals)¹⁴⁵.

The the classification of “animal nature disgust” was driven by the motivation to include four non-food contamination domains such as inappropriate sexual acts, poor hygiene, death, and violations of the body "envelope”(e.g. tissue cuts by stabbing, mutilations), which could not fit with the previous definition of core disgust. This classification changed the focus of disgust contaminations from the mouth to the whole body in general. Indeed, all of the four domains have a potential source of biological contagion and infection (e.g., venereal diseases from sex, or skin-to-skin or hair-to-hair infection from parasites on an unclean person). Rozin and Fallon concluded the potential elicitors of animal nature disgust as “anything that reminds us that we are animals elicits disgust”¹⁴⁷, indicating that just like other animals, humans must eat, excrete, and have sex. Envelope violations and death are uncomfortable reminders of our animal vulnerability. Therefore any contact with corpses or odors of decay is considered disgusting. Hygienic rules govern the proper use and maintenance of the human body. Failure to meet these culturally defined standards places a person with the same hygiene level of animals (often viewed as dirty), and therefore an unclean person can be seen as disgusting. In this view, disgust serves to ‘humanize’ our animal bodies¹⁴⁷.

Finally, the classifications of interpersonal disgust and moral disgust were driven by human cultural evolution. Interpersonal disgust is classified as a direct or indirect contact with undesirable people or simply strangers. It clearly discourages contact with other human beings who are not intimates. In fact, there is an established evidence for peoples’ aversion to contact with items (e.g. possessions, utensils, clothing, cars, and rooms) that are used by unknown or undesirable persons^155,156. This interpersonal aversion can reduce an infection risk, and therefore, it can serve the purpose of maintaining social distinctiveness and social hierarchies. For instance, it is held that interpersonal contagion is a major feature of the Hindu society in India, as it serves a basis for the maintenance of the caste system^157,158. Although a full focus is provided later in the text (section 2.2.3), in general,

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moral disgust involves the judgment of others regarding moral values. When violations of morality are detected, they induce moral disgust; a response suggested being comparable to earlier forms of disgust (see¹⁸⁹ for a review). Disgust of this kind is commonly investigated by the presentation of short stories to people and analyzing their subsequent evaluations. Moral disgust has been shown to be elicited by stories involved issues of sexuality, gore, or other instances of the misuse or abuse of human bodies¹⁶⁰, and are thus consistent with animal nature disgust. However, it was also shown to be elicited by issues that do not involve the body at all such as betrayal, hypocrisy, and racism^161–163. Although the disgust classification of Rozin^145,147 is considered the fundamental base for disgust researchers, further classifications were developed recently, offering to provide a better logical assortment. For instance, it has been suggested that the broad range of core, animal nature, and interpersonal disgust elicitors can be grouped into one larger category, as a pathogen avoidance system^164,165. Consistent with this view, Rubio-Godoy, Aunger, and Curtis¹⁶⁶ suggested that disgust is a neural system ‘‘evolved to detect reliable signals co-occurring with disease-causing infectious agents, which stimulates avoidance responses and/or other behaviors that tend to decrease the risk of disease’’. Also, based in part on evolutionary considerations, Tybur suggested modifying the classification of interpersonal and moral elicitors of disgust into two categories of sexual (mate selection) and moral (including potentially all moral violations) violations¹⁶⁴.

One of renowned recent classification was provided by Chapman, who offer to distinguish between two types of disgust: physical and moral disgust¹⁴⁶. Physical disgust refers to disgust elicited by the rather concrete assortment of stimuli and therefore include core disgust, interpersonal disgust, sexual disgust, and blood-injury disgust. Although Chapman’s classification is similar to Rozin’s view as it considers distaste as the origin of disgust, it also diverts from Rozin’s view by several aspects. First, Chapman redefines interpersonal disgust as a type of disgust that is elicited by contact with an unfamiliar, unhygienic, or diseased person. Second, Chapman uses contact-based reasoning to dissociate animal-reminder disgust into sexual and blood-injury disgust. Accordingly, sexual disgust relates to the avoidance of sexual contact with partners, who are undesirable from an evolutionary perspective. Whereas, blood-injury disgust relates to the avoidance of a potential, infectious contact with spilled blood¹⁶⁷ from injuries (violations of outer body envelope).

Although distaste can be observed in many animals (described above), it is less clear, however, whether physical disgust exists beyond our species. Whereas some animals do not show the same aversion to disease vectors (e.g., feces) that humans do¹⁴⁷, other animals show a variety of disease-avoidance behaviors¹⁶⁸. This inconsistency was suggested to occur due to different levels of disease

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risks that is faced by different species. Accordingly, some species are more vulnerable to disease than others, and therefore, they might have a greater need for disgust¹⁴⁶.

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