This chapter reviews research from psychological science on the conditions under which groups make optimal judgements, a subject area often called “wisdom of crowds”. It concludes that good judgements are more likely when the members of a group are selected for expertise, there is cognitive diversity among the members, they make independent judgements which are then aggregated and there is opportunity for sharing information and discussion. When the methods that scientists use to establish deliberative consensus are evaluated against these conditions, none meet them all, but some (Delphi studies and nominal group technique) are better than others.