Deliberation and the Wisdom of Crowds

Does pre-voting group deliberation increase majority competence? To address this question, we develop a probabilistic model of opinion formation and deliberation. Two new jury theorems, one pre-deliberation and one post-deliberation, suggest that deliberation is beneficial. Successful deliberation mitigates three voting failures: (1) overcounting widespread evidence, (2) neglecting evidential inequality, and (3) neglecting evidential complementarity. Simulations and theoretic arguments confirm this. But there are five systematic exceptions where deliberation reduces majority competence, always by increasing failure (1). Our analysis recommends deliberation that is 'participatory', 'even', but possibly 'unequal', i.e., that involves substantive sharing, privileges no evidences, but possibly privileges some persons