In-Group bias
Etymology
Sociologist William Graham Sumner coined "in-group" and "out-group" in 1906. Psychologist Henri Tajfel showed in the 1970s how little it takes to trigger favoritism: even people sorted into meaningless "minimal groups" by coin toss started allocating rewards to their own side.
noun · /IN-groop BY-us/
The tendency to favor, trust, and excuse members of your own group over outsiders.
What it names
How a sense of "us" quietly bends judgment — the same act reads as loyalty in an ally and treachery in a rival.
Example
In-group bias let the committee wave through a weak proposal from a familiar department it would have grilled from anyone else.
Related idea
Halo effect generalizes from one good trait to a whole person; in-group bias generalizes goodwill from a shared label to whoever wears it.