Fairness in group identification

We study the problem of classifying individuals into groups, using agents' opinions on who belong to which group as input. Our focus is on the rules that satisfy equal treatment of equals, a minimal fairness property, in addition to independence of irrelevant opinions and non-degeneracy. We show that a rule satisfies the three axioms if and only if it is the liberal rule, a strong one-vote rule, a one-row rule, or a one-column rule. The last three families of rules can be ruled out by simple, intuitive properties. Thus, invoking equal treatment of equals, which is substantially weaker than symmetry, we obtain a characterization of the liberal rule.