stereotype, in psychology, a fixed, oversimplified, and often biased belief about a group of people. Stereotypes are typically rationally unsupported generalizations, and, once a person becomes accustomed to stereotypical thinking, he or she may not be able to see individuals for who they are. Stereotypes can legitimize hostility against a whole social group. In addition, because stereotypes are ingrained in the culture—people begin learning stereotypes during childhood—they tend to signal which social groups are presumably appropriate targets for relieving individual frustration.

(Read Steven Pinker’s Britannica entry on rationality.)