The probability of inconsistencies in complex collective decisions

Many groups make decisions over multiple interconnected propositions. The doctrinal paradox or discursive dilemma shows that propositionwise majority voting can generate inconsistent collective sets of judgments, even when individual sets of judgments are all consistent. I develop a simple model for determining the probability of the paradox, given various assumptions about the probability distribution of individual sets of judgments, including impartial culture and impartial anonymous culture assumptions. I prove several convergence results, identifying when the probability of the paradox converges to 1, and when it converges to 0, as the number of individuals increases. Drawing on the Condorcet jury theorem and work by Bovens and Rabinowicz (2001, 2003), I use the model to assess the truth-tracking performance of two decision procedures, the premise- and conclusion-based procedures. I compare the present results with existing results on the probability of Condorcets paradox. I suggest that the doctrinal paradox is likely to occur under plausible conditions.The author wishes to express his gratitude to Luc Bovens, Matthew Braham, Steven Brams, Bruce Chapman, Philip Pettit, Wlodek Rabinowicz and two anonymous reviewers for helpful comments or discussion. A previous version of this paper was presented at the 2002 Annual Meeting of the European Public Choice Society, Belgirate, Lago Maggiore, Italy, April 2002, and at the Sixth International Meeting of the Society for Social Choice and Welfare, held at the California Institute of Technology, Pasadena, California, July 2002.