Ingroup bias rises when outgroups are similar and ingroup identification is strong.

Evidence

Supplementing these core ideas, Tajfel and Turner (1986) also proposed three types of variables that would be likely to affect intergroup differentiation: people should be identified with the ingroup; the situation should permit intergroup comparisons to be made; and outgroups should be comparable (i.e. similar or proximal). Generally, pressures for distinctiveness and subsequently enhanced levels of ingroup bias should increase with outgroup comparability. — , Rudman, Laurie A - On the Nature of…, 432

Categorization

• corroborated: 2 independent passages support this claim — , Rudman, Laurie A - On the Nature of…, 432
• XREF: connects to: Outgroup salience boosts conscious ingroup identification when the ingroup is already salient. — Brandon Randolph-Seng, Darcy A. Reich, and Kenneth G. DeMarree, On the Nonconscious Antecedents of…, 47