%0 Journal Article %T Fully Bayesian Aggregation %+ Centre National de la Recherche Scientifique (CNRS) %+ Paris School of Economics (PSE) %+ Centre d'économie de la Sorbonne (CES) %A Dietrich, Franz %Z The research was supported by the French National Research Agency through three grants (ANR-17-CE26-0003, ANR-16-FRAL-0010 and ANR-17-EURE-0001). %Z An earlier version was titled ‘The Rational Group’. https://hal.archives-ouvertes.fr/halshs-02431868v1 %< avec comité de lecture %@ 0022-0531 %J Journal of Economic Theory %I Elsevier %V 194 %P 105255 %8 2021 %D 2021 %R 10.1016/j.jet.2021.105255 %K rational group agent %K uncertainty %K preference aggregation %K opinion pooling %K values aggregation %K static versus dynamic rationality %K expected-utility hypothesis %K Bayesianism %K group rationality versus Paretianism %K spurious unanimity %K ex-ante versus ex-post Pareto %K rational group agent %Z Humanities and Social Sciences/Philosophy %Z Humanities and Social Sciences/Economics and Finance %Z Humanities and Social Sciences/Political scienceJournal articles %X Can a group be an orthodox rational agent? This requires the group's aggregate preferences to follow expected utility (static rationality) and to evolve by Bayesian updating (dynamic rationality). Group rationality is possible, but the only preference aggregation rules which achieve it (and are minimally Paretian and continuous) are the linear-geometric rules, which combine individual values linearly and combine individual beliefs geometrically. Linear-geometric preference aggregation contrasts with classic linear-linear preference aggregation, which combines both values and beliefs linearly, but achieves only static rationality. Our characterisation of linear-geometric preference aggregation has two corollaries: a characterisation of linear aggregation of values (Harsanyi's Theorem) and a characterisation of geometric aggregation of beliefs. %G English %2 https://hal.science/hal-03194928/document %2 https://hal.science/hal-03194928/file/Dietrich-FullyBayesianAggregation.pdf %L hal-03194928 %U https://hal.science/hal-03194928 %~ SHS %~ UNIV-PARIS1 %~ ENS-PARIS %~ ENPC %~ PSE %~ CNRS %~ EHESS %~ CES %~ AO-ECONOMIE %~ PARISTECH %~ AO-SCIENCEPOLITIQUE %~ AO-PHILOSOPHIE %~ PSL %~ INRAE %~ PSE-POST-PRINT %~ ENS-PSL %~ ANR