Outgroup Homogeneity
Outgroup Homogeneity Effect · Outgroup Homogeneity Bias
The tendency for the perception of out-group members as more similar to one another than are in-group members, e.g. "they are alike; we are diverse."
Origin
Psychologists George Quattrone and Edward E. Jones demonstrated the effect in a 1980 study at Princeton University, published in the Journal of Personality and Social Psychology. They showed that students perceived members of a rival university as more similar to one another than members of their own school. Subsequent work by Patricia Linville, Jones, and others through the 1980s formalized the concept and explored its roots in differential familiarity and social categorization.
Everyday Use
Fans of your favorite team see each individual player's personality. Fans of the rival team? "They're all the same." We naturally perceive rich diversity within our own groups while viewing outsiders as an undifferentiated mass — and that's the root of a lot of stereotyping.