Gray-zone categories are defined by double negation, which naturally invites family-resemblance structure.

Evidence

This arises because by definition the gray zone is not the positive pole AND not the negative pole. Gray zone concepts are often hybrid structures because between these two nots, one might use family resemblance structures. — Gary Goertz, Social Science Concepts and Measurement, 1066

Categorization

• DEFINE: Explains why negation proliferates in gray-zone definitional lists: a category that is neither pole is built from two ‘nots’.