(The Impossibility of) Deliberation-Consistent Social Choice

The forthcoming article “(The Impossibility of) Deliberation-Consistent Social Choice” by Tsuyoshi Adachi, Hun Chung, and Takashi Kurihara is summarized by the authors below.

There is now a growing consensus among democratic theorists that ‘deliberation’ and ‘aggregation (or voting)’ have their own respective virtues and that each plays an important role in the democratic process that cannot be properly reduced to the role performed by the other. Many people now think that in order to achieve democratic legitimacy/justification of the resulting outcomes, democratic institutions should incorporate both ‘democratic deliberation’ and ‘aggregative voting’ into its process, where people “first talk, then vote.” (Goodin 2008: ch. 6)  

Suppose then we have a two-stage democratic process in which people deliberate and persuade one another by exchanging reasoned arguments in the first deliberative stage, and then vote for the final outcome in the second aggregative stage with their post-deliberation preferences. What is the proper normative relationship between the first deliberation stage and the second voting stage? In this paper, we propose the following normative condition: 

• NNRD (Non-Negative Response toward Democratic Deliberation): If some individuals, through democratic deliberation (in the first stage), change their preferences toward other individuals’ preferences, then the result of the social choice rule (in the second voting stage) should not make everybody who has successfully persuaded others through reasoned deliberation worse-off than what they would have achieved without deliberation. 

NNRD characterizes what we believe to be the proper normative relationship between the two democratic stages of deliberation and voting. If NNRD is violated, then people can be made worse-off by persuading others via democratic deliberation. This disincentivizes people to deliberate, which defeats the very purpose of introducing a separate stage of deliberation into our democratic processes. Hence, it is important for any two-stage democratic process that involves both deliberation and voting to satisfy NNRD. Furthermore, it turns out that NNRD is weaker than strategy-proofness and is logically implied by it. (Proposition 1) So, if we wish our democratic processes to avoid giving people incentives to strategically misrepresent their preferences, then they must at a minimum satisfy NNRD. 

We prove that there exists no social choice rule that simultaneously satisfies NNRD along with the Weak Pareto and the No Universal Vetoer axioms. The Weak Pareto axiom expresses the democratic requirement to respect citizens’ unanimous preferences, which is a very minimal notion of popular sovereignty. The No Universal Vetoer axiom expresses our democratic commitment to political equality by not conferring too much arbitrary decision-making power on a single individual. Our impossibility theorem shows that these three normative principles cannot be simultaneously incorporated into a two-stage democratic procedure that involves both deliberation and voting.      

Then, how might we escape the impossibility result? It turns out that if we relax Weak Pareto to Top Unanimity—which requires the social choice rule to choose the alternative that is unanimously top-ranked, whenever there exists such an alternative—then we get a possibility result (Proposition 4). However, by giving up Weak Pareto, the democratic process can no longer eliminate dominated and unanimously dis-preferred alternatives that have been identified through democratic deliberation in the first stage. Similarly, if we relax No Universal Vetoer to No Dictatorship, then we again get another possibility result (Proposition 5). However, the cost here is that we must allow a single individual, who (despite not being a full-fledged dictator, who nonetheless) can always get either their best or second-best alternative regardless of the preferences of other individuals, which goes against our normative commitment to political equality.  

In short, our paper formally demonstrates that there are virtually no aggregation rules that can properly accommodate the results of such successful deliberation and at the same time respect deliberative democracy’s ideal of unanimous consensus and political equality. We can escape the impossibility result by relaxing the axioms. However, each potential escape route necessarily compromises some core normative value of democracy. 

About the Authors: Tsuyoshi Adachi is an Associate Professor of Political Science and Economics at Waseda University, Hun Chung is an Associate Professor of Political Science and Economics at Waseda University, and Takashi Kurihara is an Associate Professor of Political Science and Economics at Tokai University. Their research “(The Impossibility of) Deliberation-Consistent Social Choice” is now available in Early View and will appear in a forthcoming issue of the American Journal of Political Science.