Parasites, minds and cultures

Parasites have had profound effects on human evolution. Recent research implicates the existence of a set of psychological adaptations that serve as a first line of behavioural defence against contact with parasites – the ‘behavioural immune system’. The ordinary operation of the behavioural immune system has provocative implications for many different kinds of phenomena that are of interest to psychologists – including stigmatisation and prejudice, physical attractiveness and mating behaviour, and the origins of cultural diversity.

The littlest of things can have huge evolutionary significance. Before publishing The Origin of Species Charles Darwin spent years studying barnacles. If Darwin were alive today,we suspect that he would be mightily impressed by what we now know about the evolutionary impact of much smaller and more ancient things: viruses, bacteria, protozoa and intestinal worms that parasitise bigger organisms.

Where there is life, there are parasites – in immeasurable abundance. These parasites can seriously impair the health and reproductive fitness of the organisms that they infect. All living animals – including humans – are around today because their ancestors evolved ways to elude parasites, generation after generation. This evolutionary process has left huge footprints that researchers are just starting to discern (Ridley, 1993; Zimmer, 2000; Zuk, 2007).

The evolution of anti-parasite defence systems
Parasites don’t look or act like lions or tigers, so people don’t usually think of them as predators. But, in a sense, they are. Parasites attach themselves to the body of a host and exploit bodily resources in order to reproduce. In doing so, they can harm that host, sometimes lethally. As a consequence, host species have evolved elaborate anti-parasite defence systems.

We are all familiar with one of these defence systems: the immune system. The immune system is an amazingly sophisticated suite of adaptations, designed by natural selection to detect parasites that intrude on our bodily tissues and – once those parasites are detected – to mobilise physiological means of repelling, killing or neutralising them.

While an immune system has undeniable benefits, it has undeniable drawbacks. Mounting an immune response consumes considerable metabolic resources, which may result in temporary debilitation (e.g. fatigue, exhaustion) while the parasitic infection is being fought. Specific kinds of immune responses (e.g. fever) can be further debilitating. Most importantly, the immune system is incapable of the simplest form of defence: preventing parasites from coming into contact with the body in the first place.

It has thus been suggested that animals evolved an additional system of defence that enables them to physically avoid germy things and other infected hosts. This system is designed to employ perceptual cues (appearance, odour, etc.) to detect the presence of infectious parasites in other things – including other individuals. In some animals – including humans – the detection of such cues may trigger aversive emotional and cognitive responses that motivate behavioural avoidance. This behavioural mechanism offers a first line of defence against disease-causing parasites and hence has been called the ‘behavioural immune system’ (Schaller & Duncan, 2007).

Psychological implications of the behavioural immune system
Lots of animal species show evidence of perceptual sensitivity to cues of parasitic infection in other members of their species and of consequent behavioural avoidance (Goodall, 1986; Kavaliers & Colwell, 1995; Kiesecker et al., 1999). Humans are no exception. Recently, there has emerged a body of research exploring the implications of the behavioural immune system for human emotion, cognition and behaviour. For instance, there is evidence suggesting that the emotion of disgust evolved to serve as an affective signal of parasite infection (Curtis et al., 2004; Oaten et al., 2009). This line of evidence not only has implications for psychologists’ understanding and measurement of disgust, but also may help to explain why feelings of disgust influence moral judgements and interpersonal relations (e.g. Tybur et al., 2009).

Additional implications are emerging as well. In the sections that follow, we briefly review three particularly intriguing sets of findings that illustrate the wide-ranging psychological implications of the behavioural immune system. One set of findings pertains to the psychology of stigmatisation and prejudice; a second set pertains to the psychology of physical attractiveness and mating behaviour; and a third set bears on the origins of culture and cross-cultural differences.

Stigmatisation and prejudice
Because most parasites are virtually invisible, people must rely on superficial cues (e.g. anomalous physical features) to detect their presence. Because cues are imperfectly correlated with parasitic infection, there emerges a signal-detection problem in which errors are inevitable. Any attempt to limit the number of ‘false-positive’ errors (erroneously inferring the presence of parasites where there are none) inevitably leads to an increase in ‘false-negative’ errors (erroneously inferring the absence of parasites where, in fact, they exist), and vice versa.

How is this signal-detection problem resolved? An answer is provided by the ‘smoke detector principle’ (Nesse, 2005).A smoke detector is typically calibrated to be supersensitive to anything that superficially resembles smoke, in order to minimise the likelihood of failing to register the presence of a house fire (a very costly false-negative error). The inevitable consequence – which people happily tolerate – is lots of (relatively less costly) false-positive errors: the smoke detector may sound its alarm anytime someone is harmlessly braising a steak or boiling a pot of pasta.

A similar set of functional priorities applies to the behavioural immune system. In order to avoid the highly costly consequences that may follow from contact with parasites (e.g. illness, death), the system is calibrated to be supersensitive to superficial cues (e.g. a wide range of morphological or behavioural anomalies) connoting the possible presence of parasites. The result is that the behavioural immune system may sound its alarm (and trigger aversive affective, cognitive and behavioural responses) whenever a person perceives someone else whose superficial physical appearance or behaviour deviates from whatever prototype people perceive to be ‘normal’.

There is another important consideration to keep in mind. Just as the activation of the ‘real’ immune system has costs, the activation of the behavioural immune system has costs as well (e.g. consumption of metabolic resources). Therefore, the behavioural immune system is likely to be especially supersensitive and especially likely to trigger aversive responses when its benefits are especially likely to outweigh its costs; that is, whenever perceivers are, or merely perceive themselves to be, especially vulnerable to the transmission of disease.

This line of reasoning has two broad implications for our understanding of stigmatisation and prejudice. First, the ordinary operation of the behavioural immune system may contribute to the stigmatisation of people whose appearance deviates from some subjective sense of normalcy. Second, prejudicial responses to these people are likely to be exaggerated under conditions in which perceivers are (or merely perceive themselves to be) especially vulnerable to parasite transmission.

This analysis thus helps us understand why people suffering from some diseases (such as leprosy) have historically been more highly stigmatised than people suffering from other diseases (which may be more virulent and infectious, but are associated with less overt morphological anomalies).

More provocatively, this analysis suggests that psychologically similar prejudicial responses may be directed against individuals who aren’t actually suffering from any infectious disease whatsoever – and that these prejudicial responses vary depending on the extent to which perceivers feel vulnerable to parasite transmission.

In our own labs, we have conducted studies to assess the extent to which people are especially likely to implicitly associate the concept ‘disease’ (as well as other aversive cognitions) with specific categories of people. One set of studies implicated the behavioural immune system in implicit prejudices directed against people with superficial facial birthmarks and physical disabilities (Park et al., 2003; Schaller & Duncan, 2007). Another set of studies implicated the behavioural immune system in prejudicial responses to obesity (Park et al., 2007). Among other findings, we discovered that when the threat of parasite transmission is made temporarily salient (e.g. with a brief slideshow depicting germs and their presence all around), people are especially likely to implicitly associate obese individuals with the semantic concept ‘disease’. These findings not only help to illuminate the causes of weight-based prejudice, they also illustrate the point that the behavioural immune system responds not to rational assessments of parasite infection (after all, parasite infection is more likely to lead to weight loss than weight gain), but instead to relatively crude and wide-ranging perceptual cues.

Ethnocentrism and xenophobia also appear to be rooted, in part, in the irrational operation of the behavioural immune system. People who feel especially vulnerable to parasite transmission are especially likely to favour contact with familiar rather than foreign peoples (Faulkner et al., 2004). A particularly provocative finding was reported by Navarrete et al. (2007): women in the first term of pregnancy – whose bodies are naturally immunosuppressed – show especially high levels of xenophobia and ethnocentrism.