Group Attribution Error
The tendency to believe either that the characteristics of an individual group member are reflective of the group as a whole, or that a group's decision outcome must reflect the preferences of individual group members.
Origin
Psychologists Scott T. Allison and David M. Messick introduced the term in their 1985 paper "The Group Attribution Error" in the Journal of Experimental Social Psychology. They drew on earlier work by Ruth Hamill, Richard Nisbett, and Timothy Wilson, who in 1980 first studied how people generalize from atypical individual cases to entire groups. Allison and Messick framed the bias as a group-level analogue of the fundamental attribution error.
Everyday Use
You meet one rude employee and decide the whole company has terrible service, or you assume every member of a committee personally supports its decision. We constantly leap from "one person did X" to "they're all like that" — and it fuels stereotypes, partisan thinking, and workplace friction.