When it comes to avoiding infection, a growing body of evidence suggests we don't just have a physiological immune system, we also have a behavioural immune system - one that alerts us to people likely to be carrying disease, and that puts us off interacting with them. Indeed, there's research showing that people who are more fearful of disease tend to hold more xenophobic attitudes and to display greater prejudice towards people with outwardly visible disabilities. Now Chad Mortensen and his co-workers have extended this line of research by showing that a disease-themed slide show makes people feel less sociable and extravert, and primes their motor system for repelling other people.
In the first study, half of 59 participants watched a disease and infection-themed slide show before completing a measure of their own personality. The other participants watched a slide show about architecture before doing the same. The researchers took pains to conceal the true purpose of the study. They asked participants to rate the slide shows' usefulness for another project and they had them answer irrelevant questions. The key finding was that participants who watched the disease slide show subsequently rated themselves as less extravert than did the control participants. Also, among those participants who scored highly on a measure of fear of disease, those who watched the infection slide show rated themselves afterwards as less open to experience and less agreeable. Taken altogether this suggests that reminders of disease makes us view ourselves as less outgoing and gregarious, especially if we're the kind of person who's already fairly neurotic about infection.
If these effects are real, you'd expect them to have some effect on actual behaviour. The second study tested that by having participants watch one of the slide shows before completing a computer task. The task involved faces and shapes flashing on a screen and participants responding with a button press that required either an extension or contraction of the arm. The take-home finding here was that participants who watched the disease slide show were quicker at the button presses that required them to extend their arm - the same muscle action that would be required to push someone away. This effect was particularly strong among those participants who were more scared of infection. Again, cover stories were used to conceal the true purpose of the study.
'...It appears that humans have evolved a mechanism that responds to environmental cues of disease and modulates attitudes and behaviours in functionally appropriate ways,' the researchers said.
Looking to the future, Chad Mortensen and his colleagues added that it would be interesting to see if there could be a reverse effect in conditions in which risk of infection appeared to be absent. In this case, people normally afraid of infection might become particularly extravert and sociable.
Mortensen, C., Becker, D., Ackerman, J., Neuberg, S., & Kenrick, D. (2010). Infection Breeds Reticence: The Effects of Disease Salience on Self-Perceptions of Personality and Behavioral Avoidance Tendencies. Psychological Science, 21 (3), 440-447 DOI: 10.1177/0956797610361706
Related open-access article in The Psychologist magazine: 'Parasites, minds and cultures'.