Fair and efficient collective decisions via nondeterministic proportional consensus

Abstract

Are there collective decision methods which (i) give everyone, including minorities, an equal share of effective power even if voters act strategically, (ii) promote consensus and equality, rather than polarization and inequality, and (iii) do not favour the status quo or rely too much on chance? We show the answer is yes by describing two nondeterministic group decision methods, one based on automatic bargaining over lotteries, the other on conditional commitments to approve compromise options. Our theoretical analysis and agent-based simulation experiments show that, when these group decision methods are used, majorities cannot consistently suppress minorities as with deterministic group decision methods, proponents of the status quo cannot block decisions as in other consensus-based approaches, the resulting aggregate welfare is comparable to that provided by other common voting methods, and the average amount of chance employed by the method is lower than for other nondeterministic methods. Our results suggest that the welfare costs of fairness and consensus are small compared to the inequality costs of majoritarianism. [PDF contains the 20 pages manuscript and 61 pages of appendices]