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Perspectives in Biology and Medicine 44.1 (2001) 17-31

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Dirt, Disgust, and Disease: is hygiene in our genes?

Valerie Curtis* and Adam Biran


. . . pus, vomit, urination, menstruation, sexual fluids and so on--all substances and acts that, for some reason, many cultures tend to see as repellent and, despite their constant presence in human life, as abnormal.

--A. K. Reinhart (1990)


Anthropologists have long puzzled over why certain objects and activities are avoided, reviled, or proscribed in many cultures. Numerous theories have been proposed, but as Reinhart (1990) suggests above, a full explanation remains elusive. Psychologists recently have begun to explore the nature of the revulsion that is occasioned by the sight of excreta, rotten food, slime, and bugs. They have described and categorized the emotion of disgust and have even proposed a location in the brain where disgust may be seated. However, the total body of research into disgust is so scant that it has been described as the "forgotten emotion of psychiatry" (Phillips et al. 1998).

Our interest in disgust has its roots in a decade of work exploring hygiene behavior in Africa, India and Europe. The failures of the health education [End Page 17] approach in promoting hygiene has drawn attention to the need to understand existing motivations and practices more fully. Although there is much variation in the behaviors that are considered acceptable and appropriate in different societies, we found signs of a consistent pattern. We found that hygiene was important to all of the people that we worked with, and that hygienic behavior often was motivated by the desire to avoid or remove things that were found disgusting. Review of the anthropological, psychological, historical, and medical literature suggests a wide variety of explanations for hygienic behavior. However, few writers offer explanations for the origins of hygiene or consider how it might be related to the disgust emotion. This paper explores the nature of disgust and argues that it can best be understood as a mechanism for defense against infectious disease.

What Is Disgust?

Disgust is a powerful emotion and is thought to be a human universal. Darwin (1872) counted it as one of the six basic emotions. The manifestations of disgust include a particular facial expression (wrinkling of the nose, pulling down the corners of the mouth), characteristic neurological signs (lowered blood pressure, lowered galvanic skin response, and nausea) and characteristic actions (stopping, dropping the object of disgust, shuddering or saying "yuk!") (Rozin et al. 1993). The facial expression of disgust has been found to be recognizable across cultures (Ekman and Friesen 1986; Mesquita and Frijda 1992). Disgust apparently is distinguishable from fear in that disgust involves a suspension of activity, while fear heightens activity in preparation for fight or flight (Phillips et al. 1998). Recent magnetic resonance imaging studies have proposed a specific neurological substrate for disgust, located in the anterior insular cortex (Phillips et al. 1997).

Elicitors of Disgust

Though the details of what constitutes a disgusting stimulus may vary to some degree from culture to culture (Davey et al. 1998) and from individual to individual, there appear to be some prototypical objects of disgust. Phillips and colleagues (1998) suggest that these are waste products of the human body, while Rozin and Fallon (1987) see the key source of disgust as "the prospect of [oral] incorporation of an offensive object." Rozin and Fallon continue: "The offensive objects are contaminants; that is, if they even briefly contact an acceptable food they tend to render that food unacceptable." According to Rozin, substances of animal origin, poor hygiene, violations of the body envelope, and death are disgust stimuli. Disgust is also elicited by physical contact with unpleasant or unknown people (Rozin and Fallon 1987). Furthermore, disgust appears to have a cultural domain and can be elicited by immorality and violations of social rules (Miller 1997; Rozin et al. 1999b). [End Page 18]

Our program of qualitative field work exploring the motivation for hygiene behavior generated sets of objects and events that were found to be disgusting from five...