We are prejudiced against all kinds of other people, based on superficial physical features: We react negatively to facial disfigurement; we avoid sitting next to people who are obese, or old, or in a wheelchair; we favor familiar folks over folks that are foreign. If I asked you why these prejudices exist and what one can do to eliminate them, your answer probably wouldn't involve the words "infectious disease." Perhaps it should.

What does infectious disease have to do with these prejudices? The answer lies in something that I've come to call the "behavioral immune system." The behavioral immune system is our brain's way of engaging in a kind of preventative medicine. It's a suite of psychological mechanisms designed to detect the presence of disease-causing parasites in our immediate environment, and to respond to those things in ways that help us to avoid contact with them. This has many important implications – for prejudice, for sexual attraction, for social interaction, and even for the origins of cultural differences. (And, yes, for health too.)

It makes immediate sense that people would develop aversions against people who actually have infectious diseases. But why does it also lead to these aversions to perfectly healthy people? Because it's impossible to directly detect the presence of bacteria and viruses and other microscopic parasites; and so we're forced to use crude superficial cues. Consequently, we make mistakes. Some of those mistakes lead to the irrational avoidance of things (including people) that pose no infection risk at all.

Here's an example: Animal feces is loaded with parasites that can make you ill. So if something looks like a pile of dog poop, you probably won't eat it. That's smart. But what if I took some delicious chocolate fudge and molded it into the shape of poop? Research by Paul Rozin and his colleagues shows that a lot of people still won't eat it – even though they know it's fudge! These people aren't responding to any rational appraisal of infection risk; they are responding – automatically and aversively – to appearances.

The same principle applies in our interactions with other people. Our minds seem to be on the lookout for anything that looks like it might be a symptom of infection. But there are lots of different kinds of infections and lots of different kinds of symptoms; consequently, anything anomalous about a person's appearance or behavior (anything that deviates from our subjective sense of how a "normal" person looks and acts) can automatically trigger aversive emotional, cognitive, and behavioral responses. In recent years, my collaborators and I have found that these disease-avoidant psychological processes contribute to prejudices against people who are disabled, obese, or old. These same processes also contribute to ethnocentrism and xenophobia.

One interesting implication is that these prejudices are exaggerated when people feel vulnerable to infection, whereas the prejudices are reduced among people who feel relatively safe. This implication was neatly demonstrated in a study published in Evolution and Human Behavior, which examined prejudices in pregnant women. A woman's immune system is suppressed during the first few weeks of pregnancy, leaving her body more vulnerable to infection. One consequence is that women are more sensitive to sights and smells and tastes that trigger disgust. Another consequence is that, compared to women in later stages of pregnancy, women in their first trimester show higher levels of ethnocentrism and xenophobia.

The perceived threat of infection has interesting implications for sexual attraction too. For instance, it's been known for a long time that people find symmetrical faces more attractive. Why is this? One possibility is that people with symmetrical faces are not only more likely to be healthier themselves, but also more likely to produce offspring with stronger immune systems. Consistent with this reasoning, recent studies published in the Proceedings of the Royal Society and in the European Journal of Social Psychology reveal that the preference for symmetrical faces (especially opposite-sex faces) is exaggerated under conditions in which people are more keenly aware of the threat posed by infectious diseases.

Another line of work focuses on social attitudes and interactions more generally. In an article published last year, researchers at Arizona State University found that when the threat of infection was especially salient, people were less extraverted.

The implications of the behavioral immune system may also play out at a societal level, and can help to explain worldwide cross-cultural differences. If people behave differently depending on their vulnerability to infection, then whole populations of people may differ, depending on the local prevalence of parasites. Damian Murray and I found that in countries characterized by high levels of parasite prevalence, people are less extraverted and less open to new experiences, and also impose greater conformity pressures on one another. In collaboration with Corey Fincher and Randy Thornhill, we found that parasite prevalence was a strong predictor of collectivistic value systems. Fincher and Thornhill have other evidence of similar effects. For instance, in an article published in Biological Reviews, they report that parasite prevalence, and adaptive human responses to it, may help explain cross-national differences in political ideology and systems of governance.

Although the behavioral immune system is designed to reduce contact with infectious parasites, infections still occur. Luckily, we have the "real" immune system too, which works very hard to eliminate those parasites from our bodies after infection occurs. These two systems of anti-pathogen defense are physiologically distinct, but they influence each other in interesting ways. A recent study published in Psychophysiology showed that when people experience disgust (the emotion that signals threat of infection), there are increased markers of immunological function in people's saliva. And in an experiment that my colleagues and I published last year in Psychological Science, we found that the mere sight of other people's symptoms of sickness (sneezes, sores, rashes) led perceivers' own white blood cells to respond more aggressively to bacterial infection.

Medical scientists have been studying immunological defense for decades. By contrast, it's only in the past few years that psychological scientists have been doing research on the other immune system. Still, it's already apparent that this other immune system – the behavioral immune system – has important influences on a wide range of human affairs. In order to fully explain why people think what they think or do what they do, we may need to talk a lot more about the threat of infectious disease and the psychological processes that, for better and worse, respond to that threat.

Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.