Rational Aggregation

29 Pages Posted: 13 Dec 2001 Last revised: 17 Jun 2008

See all articles by Bruce Chapman

Bruce Chapman

Faculty of Law, University of Toronto


In two recent papers Christian List and Philip Pettit have argued that there is a problem in the aggregation of reasoned judgements that is akin to the aggregation of preference problem in social choice theory. List and Pettit prove a new general impossibility theorem for the aggregation of judgements, showing that no judgement aggregation function for a group is possible if the group seeks to satisfy certain "minimal conditions". These conditions are designed to ensure that the function is both responsive to the individually rational views of its members and collectively rational in the set of judgements it holds.

In this paper I resist the List and Pettit claim that there is the same propensity for collective irrationality in the aggregation of reasoned judgements as there is in the aggregation of preference. I argue that group decisions based on reasons are mediated by a logical structure that is lacking in group decisions based on mere preference. This additional logical structure means that the level of agreement of individual judgements on some propositions must be given a priority over comparable levels of agreement over others, a priority that is not permitted by one of the conditions imposed by List and Pettit. The need for this priority is particularly apparent, I argue, if one views the aggregation of judgement through the lens of common law decision-making.

Keywords: Social choice, judgement, Condorcet jury theorem, collective rationality, public reason

Suggested Citation

Chapman, Bruce, Rational Aggregation. Politics, Philosophy, and Economics, Vol. 1, pp. 337-54, 2002. Available at SSRN: https://ssrn.com/abstract=293126

Bruce Chapman (Contact Author)

Faculty of Law, University of Toronto ( email )

78 and 84 Queen's Park
Toronto, Ontario M5S 2C5
416-978-6911 (Phone)
416 978 2648 (Fax)

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