What is the in-group bias (in-group favouritism)?

We instinctively favour, trust and excuse people we see as part of our own group, and are warier and harsher toward outsiders.

The mechanism is rooted in social identity: belonging to a group is part of how we define and feel good about ourselves, so we unconsciously prop up "us" and downgrade "them". The grouping can be deep (nationality, religion) or utterly trivial — even being assigned a random team colour is enough to trigger favouritism.

Example: Fans of two sports teams watch the same disputed foul and each sincerely "sees" the referee as biased against their own side — the identical footage is read through loyalty, not neutrally.

Tip: Notice when "they would do that, of course" and "we'd never do that" are applied to identical behaviour — that asymmetry is the bias talking.