Despite the powers of the immune system to fight disease, there's no better defense than not getting sick in the first place, right? Washing our hands after contact with someone who has germs is a behavior most of us know can prevent infection. But shunning somebody with a sniffle altogether can also head off a nasty illness.
Scientists have begun to define some of such disease-wary actions - and the emotions and thinking behind them - as examples of what could be called a psychological immune system. A recent Current Directions in Psychological Science paper (.pdf) by psychologist Mark Schaller, PhD, of the University of British Columbia, provides a brief introduction to the fairly new concept, which he prefers to call the behavioral immune system:
The behavioral immune system consists of a suite of psychological mechanisms that (a) detect cues connoting the presence of infectious pathogens in the immediate environment, (b) trigger disease-relevant emotional and cognitive responses, and thus (c) facilitate behavioral avoidance of pathogen infection.
In other words, we're constantly on the lookout for the signs of infection in the world (such as blood, greenish or yellowish oozes, and foul smells) and also signs of sickness in the people around us (such as skin blemishes, coughs, and sneezes). Seeing these signs triggers negative emotions like disgust or anxiety, which make us more alert to the danger of infection. In turn, we seek to avoid the disgusting or anxiety-causing infection-risk by any means possible.
Schaller says that evolution probably set our behavioral immune system to be triggered according to the "smoke detector principle," which means it tends to produce many false alarms because the cost of missing something that might make us sick is so high compared to the cost of avoiding a potential threat:
The system responds to an overly general set of superficial cues, which can result in aversive responses to things (including people) that pose no actual threat of pathogen infection.
Social and evolutionary psychologists like Schaller have been conducting odd experiments detailing how easily this set of disease avoidance mechanisms can be triggered. Did you know that a bad odor makes people more likely to practice safe sex? Or that exaggerated media coverage of disease outbreaks seems to make people less outgoing (.pdf) and perhaps more prejudiced (.pdf)? Schaller's research has shown that simply looking at disease-y photos immediately kicks the immune system into a higher gear (pdf).
Photo by Oscar Gustave Rejlander from Charles Darwin's The Expression of the Emotions in Man and Animals