The Evolution of Anti-Parasite Defence Systems
Mounting an immune response consumes considerable metabolic resources, which may result in temporary fatigue even exhaustion while the parasitic infection is being fought. It has thus been suggested that animals evolved an additional system of defence: 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’…
…there is evidence suggesting that the emotion of disgust evolved to serve as an affective signal of parasite infection. 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…
The same logic applies to personality traits such as extraversion and openness to experience. These traits may confer specific kinds of benefits (contact with new friends and new technologies). But both are also likely to be associated with a specific kind of cost: greater exposure to parasites. These costs are greater in regions with a high prevalence of parasites. The implication, supported by empirical evidence, is that in regions characterised by a higher prevalence of parasites, people are less extraverted and less open to new ideas…
Collectivism is defined in part by an emphasis on conformity to existing traditions and norms, whereas individualism is defined in part by a tolerance for deviance. Individualism therefore connotes a greater risk for parasite transmission. It follows that collectivistic value systems are especially likely to emerge and persist in regions characterised by a high prevalence of parasites, whereas individualistic value systems are most likely to take hold in regions with a relatively low level of parasites.
We have focused here on how the human mind is adapted to minimise the threat of parasite infection and on the psychological consequences of those adaptations. Perhaps nothing is as humbling as learning that the most human of qualities – patriotic feelings, appreciation of beauty, sexual passion, cultural diversity – may owe their existence to tiny mindless disease-causing organisms. But by giving parasites their due attention, we are beginning to get a handle on some of the oldest questions and discovering that psychology is even more profound than it first appears.
Parasites, Minds and Cultures by Justin H. Park and Mark Schaller







