A Proof for 'Who is a J' Impossibility Theorem

In the analysis of group identification, Kasher and Rubinstein (1997), Logique Analyse 160, 385-395, have shown that any method to aggregate the opinions of a group of agents about the individuals in the group that posses a specific attribute, such as race, nationality, profession, etc., must be dictatorial or, otherwise, it must violate either consensus or independence. This result is known in the literature as 'Who is a J' impossibility theorem. This note enhances slightly the result by weakening the axiom consensus, and it offers a direct proof of the theorem based on the structure of the family of decisive coalitions.