Objective category overlap in a population can be measured and varies

Evidence

Actual versus Perceived Category Overlap The starting point for Roccas and Brewer’s (2002) conceptualization of social identity complexity is similar to that of crossed categorization, namely that many social categories in the real world naturally cut across each other; in fact, they differ in the extent to which they do so. In other words, it is possible to perceive the many and varied social categories individuals belong to as differing in the extent to which they overlap. For example, since a large proportion of British people are Christian, there is relatively high overlap between these two social categories. But a relatively smaller proportion of British people are Jewish, thus reflecting relatively lower overlap. — Roderick M. Kramer, Geoffrey J. Leonardelli, Robert W. Livingston, Social Cognition, Social Identity, and…, 521

Categorization

• DEFINE: Introduces the actual-overlap axis that distinguishes Roccas and Brewer’s model from plain crossed categorization, anchored in England’s Christian-majority versus Jewish-minority base rates.
• corroborated: 2 independent passages support this claim — Roderick M. Kramer, Geoffrey J. Leonardelli, Robert W. Livingston, Social Cognition, Social Identity, and…, 521