Politicians frequently draw on data and expert analysis in public debates. The use of ‘hard facts’ and statistics can be an important rhetorical device, lending credibility to a claim, or underlining the urgency of a problem (Edelman 1977). Given the highly adversarial nature of party politics, and its focus on rhetoric rather than output, we would expect knowledge to be marshalled primarily to substantiate competing claims. Expert knowledge is likely to perform the function of backing up particular programmes and policies, or underpinning critiques of rival parties' policies. It may also play a legitimizing function, drawn on to demonstrate the capacity of a government or political party to make sound decisions (or, by extension, to question the capacity of opposition parties to do so). Since party politics is essentially about talk rather than action (Brunsson 2002), knowledge is unlikely to perform an instrumental function, except insofar as analyses of public opinion are used to adjust strategies of political mobilization (a form of strategic political knowledge that is outside the scope of this book).

This chapter therefore focuses on the substantiating function of knowledge in public debates. It argues that two main sets of factors are likely to shape the role of expert knowledge in party political debate. The first, considered in the initial section of the chapter, relates to features of the policy area and the policy preferences being put forward.