The behavioral immune system is the first line of defense against potentially contagious diseases, before the physiological immune system. A cluster of psychological mechanisms avoids contact with contaminants: disgusting taste (e. g. sour milk), foul smell (garbage), aversive sounds (clearing throat), ugly sights (vomit), and sticky substance on fingers. This cluster was formed in our evolutionary past when groups of humans lived relatively isolated and contact with out-groups raised the chances of contact with contagious diseases which were new for the physiological immune system.
The behavioral immune system differs in strength between individuals. As an evolutionary heritage it shows clear relations with political attitudes, as US-American researchers report in the recent issue of Evolution and Human Behavior. The stronger a person’s behavioral immune system is (fear of contamination, disgust sensitivity), the stronger are conservative attitudes, like right-wing conservatism, religious fundamentalism, and ethnocentrism. (Terrizzi, J. A. Jr., et al. (2013). The behavioral immune system and social conservatism: a meta-analysis. Evolution and Human Behavior, 34, 99 108).