Besides our biological immune system, psychological research has recently found evidence that humans have a “behavioral immune system”— a tendency to avoid people who may be carrying disease. Besides avoiding obviously ill people, psychologists think humans universally tend to conform to our own “in-group” and avoid people who are different from us, because originally, we were trying to avoid infection.
This was actually a risk during our early evolution. When we were wandering hunter- gatherers and encountered another wandering tribe, the strangers might have encountered different diseases and be carrying germs to which they had acquired resistance, but we had not. This was especially true because some aspects of disease resistance are genetic, and we would have shared fewer genes with another wandering tribe than we do now with fellow city dwellers. The disease risks that separated populations may pose to each other were confirmed with a vengeance when most ...
