In-group bias (also called in-group favoritism) is the tendency to favor members of one's own group over outsiders — evaluating the same behavior more positively when performed by a group member.

What Counts as a Group

Groups can form around almost anything: nationality, race, sports team, religion, political party, employer, school, or even arbitrary lab assignments. The bias appears almost instantly once a group identity forms.

Documented Effects

• Hiring managers rate identical resumes higher when the applicant shares their alma mater
• Referees favor home teams even in objective measurements
• People attribute success to in-group members' skill and out-group members' luck

The Minimal Group Paradigm

In famous experiments, Henri Tajfel showed that people exhibit in-group bias even when randomly assigned to groups based on completely trivial criteria — like a coin flip.

Implication

Diversity in decision-making isn't just ethical — it's epistemically better. Homogeneous groups are structurally prone to in-group bias amplification.

Reference:

Wikipedia: In-Group Favoritism

https://en.wikipedia.org/wiki/In-group_favoritism