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.

Group judgements are often – implicitly or explicitly – influenced by their members’ individual expertise. However, given that expertise is seldom recognized fully and that some distortions may occur (bias, correlation, etc.), it is not clear that differential weighting is an epistemically advantageous strategy with respect to straight averaging. Our paper characterizes a wide set of conditions under which differential weighting outperforms straight averaging and embeds the results into the multidisciplinary group decision-making literature.

A normative Bayesian theory of deliberation and judgement requires a procedure for merging the evidence of a collection of agents. In order to provide such a procedure, one needs to ask what the evidence is that grounds Bayesian probabilities. After finding fault with several views on the nature of evidence (the views that evidence is knowledge; that evidence is whatever is fully believed; that evidence is observationally set credence; that evidence is information), it is argued that evidence is whatever is rationally taken for granted. This view is shown to have consequences for an account of merging evidence, and it is argued that standard axioms for merging need to be altered somewhat.

How does an individual decision maker update his or her beliefs in the light of others’ beliefs? We present an empirical investigation that varies decision makers’ access to other peoples’ beliefs: whether they know what course of action others have taken (in this case how a problem is solved) and whether they know why this course of action was taken (why a particular solution is preferred). We propose a number of process models of advice taking that do and do not accommodate the reasons given for belief (epistemic social information), and evaluate which is used through model comparison techniques.

We present a model of collective decision making in which aggregation and deliberation are treated simultaneously. Individuals debate in a public forum and potentially revise their judgements in light of deliberation. Once this process is exhausted, a rule is applied to aggregate post-deliberation judgements in order to make a social choice. Restricting attention to three alternatives, we identify conditions under which a democracy is ‘truth-revealing’. This condition says that the deliberation path and the aggregation rule always lead to the correct social choice being made, irrespective of both the original profile of judgements and the size of the electorate.

Assuming that in order to best achieve their goal, individuals adapt their behaviour to the game situation, this paper examines the appropriateness of the Berge behaviour rule and equilibrium as a complement to Nash. We define a Berge equilibrium and explain what it means to play in this fashion. We analyse the rationale of individuals playing in a situational manner, and establish an operational approach that describes the circumstances under which the same individual might play in one fashion versus another.

Hausman and McPherson defend welfare economics by claiming that even if welfare does not consist in preference satisfaction, preferences still provide good, if fallible, evidence of welfare. I argue that this strategy does not yet fully solve the problems for welfare economics stemming from the preference satisfaction theory of welfare. More work is needed to show that our self-interested preferences are sufficiently reliable, or in some other sense our best, evidence of well-being. Thus, my aim is to identify the challenges that remain and clarify what additional work is needed before Hausman and McPherson's defence of welfare economics succeeds.