Theory Dec. (2018) 85:1–4 https://doi.org/10.1007/s11238-018-9654-z Introduction to the special issue “Beliefs in Groups” of Theory and Decision Franz Dietrich 1 · Wlodek Rabinowicz 2 Published online: 20 June 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018 This symposium in the overlap of philosophy and decision theory is described well by its title “Beliefs in Groups”. Each word in the title matters, with one intended ambiguity. The symposium is about beliefs rather than other attitudes such as prefer- ences; these beliefs take the form of probabilities in the first three contributions, binary yes/no beliefs (‘judgments’) in the fourth contribution, and qualitative probabilities (‘probability grades’) in the fifth contribution. The beliefs occur in groups, which is ambiguous between beliefs of groups as a whole and beliefs of group members. The five contributions—all of them interesting, we believe—address several aspects of this general theme. Where contributions address beliefs of group members, the central question is that of belief revision: how should individuals revise their beliefs after learning those of others? This question is of obvious interest in the context of deliberation and exchange of opinions. By contrast, where contributions address beliefs of the group as a whole, the central question is that of aggregation: how should the beliefs of group members be merged into collective beliefs? The two questions are interconnected in many ways. For one, revising one’s beliefs may take the form of aggregating them with learnt beliefs of others—for instance through averaging probability assignments, something analysed in depth in the first three contributions. This approach reduces revision to aggregation. A converse reduction is also imaginable, though not common. One might argue that the right aggregate beliefs are those beliefs which would emerge as consen- sus beliefs through suitable deliberation and belief revision by the group members, be it in one revision round, finitely many revisions rounds, or a converging infinite B Franz Dietrich fd@franzdietrich.net 1 Paris School of Economics and CNRS, Paris, France 2 Department of Philosophy, Lund University, Lund, Sweden 123