Deliberation is the process through which we decide what do to, or what to believe. When we think about what to do, we are engaged in practical deliberation. Theoretical deliberation is when we think about what to believe, or about which judgement to make.

Deliberation, be it practical or theoretical, involves at least three distinct tasks. We need to find out what our options are, then to figure out what the consequences of our choices could be, and what the factors are that, together with our choices, will determine these consequences. Finally we need to weigh reasons for and against taking the various options we have.

Deliberation combines individual and social components. We can deliberate by ourselves about what to do and what to believe. But we can also do this together. We can exchange information and evaluate each other's evidence, discuss the value of achieving certain goals, and identify the obligations that bear on us.

Group deliberation often leads to group decisions, but not always. When a hiring committee has to decide on a candidate the group's opinion is formed by aggregating the judgements and/or reasons of its members. But we also deliberate in groups to change, and hopefully improve, our own, individual opinions through discussion with others. For instance, we scientists meet regularly in conferences to get feedback from others on our recent findings.

When we deliberate in groups to form or change our own opinion, we typically seek advice or receive opinions from others. We might treat them as experts, peers, or simply as additional sources of information. This of course raises interesting philosophical questions. To what extent, if at all, can this improve our individual opinions and decisions? When do we, and when should we take the opinion of others into account? Is finding consensus a desirable aim of such group deliberation? When diverging judgement will be somehow aggregated at the end of the deliberation, is consensus relevant at all?

The papers in this special issue of Economics and Philosophy tackle some of these questions regarding deliberation in social contexts. Klein and Sprenger's paper studies group deliberation and the formation of collective judgements in situations where there is uncertainty about the members’ expertise. They show analytically that in a large number of cases the so-called ‘differential weighting’ method outperforms taking a straight average of the group members’ opinion. Williamson's paper also addresses the question of theoretical deliberation. It starts by arguing that the best way to aggregate group opinion is to aggregate evidence that members have. The paper then argues that consideration on the very nature of evidence force us to revisit traditional methods for probabilistic pooling. Wallin and McElreath investigate empirically which ‘strategies for advice taking’ are used by participants in experiments on deliberation. They show that when information about reasons is made available to the participants, not only do they use it but that this significantly increases their performance on the tasks at hand. Perote-Pena and Piggins investigate cases in which a mixed model of public deliberation is truth-conducive. By mixed model we mean cases that involve both a phase of information exchange and a phase of aggregation. They show that this ‘first talk, then vote’ approach can, under some conditions, lead with certainty to correct or true group opinion.

The idea of making a special issue on deliberation was conceived in the context of a European Research Network on Rationality and Decision funded by the Netherlands Organisation for Scientific Research (NWO), and supported by grant programmes running at several other institutions (the London School of Economics, Ludwig Maximilians University Munich, University of Leuven).