Measuring Agenda-Setting Influence from Legislative Speech

Assessing which actors are influential in political debate is important for understanding the mechanisms behind legislative decision-making. Conceiving of ‘influence’ as a speaker’s ability to mould discussion of an issue towards their own framing, I propose a measurement strategy which infers influence by modelling each speech in a debate as a function of the speeches that preceded it. Intuitively, an influential speech is one that is highly predictive of other speeches that occur later in the debate, and influential legislators are those who deliver influential speeches. I apply this method to debates in the UK House of Commons from 1979 to 2018, and compare the measure to potential alternatives in a series of validation tests. I demonstrate the value of this approach by using the measure to address important questions in legislative politics. This version: April 2018. I thank Ben Lauderdale and Arthur Spirling for helpful discussions. Introduction Who is influential in political debate? The speeches that politicians make can be viewed as purposive attempts to alter the perceived issue space in such a way as to change “the interpretation of what is at stake” (Shepsle, 2003, 309) and thereby shift debate outcomes (Riker, 1986). However, we have few empirical measures for determining which actors are successful in this regard. As debate is a central feature in the work of all parliaments, understanding which legislators are most able to shape the debate agenda is crucial for addressing questions of intra-party politics, government accountability, and legislative behaviour. This paper presents an approach for measuring relative levels of agenda-setting influence amongst legislators, which I apply to speeches made by Members of Parliament (MPs) in the UK House of Commons between 1979 and 2018. Debaters may acknowledge influential speakers by explicitly referring to important speeches, or by implicitly alluding to the ideas and arguments that such speeches express. In the legislative context, explicit references are rare, but I argue that implicit endorsements of influential speakers can be revealed by studying the vocabulary used by participants in political discourse. Intuitively, influential MPs will use language that is adopted and discussed by other MPs in subsequent speeches. When another MP adopts your framing of an issue, they are implicitly indicating that your perspective is important. Influential MPs therefore literally ‘shape the debate’, as the framing they use alters the vocabulary used by other MPs in subsequent discussion. I capture this intuition by modelling the text of each speech in a debate as a function of the texts of the speeches that preceded it. At the speech-level, I use a penalised regression framework to model the words used in each speech, where the words used by all previous speeches are explanatory variables. The model implies that earlier speeches must be strongly predictive of future speeches, conditional on all other speeches in the debate, in order to be counted as influential. At the debate-level, I rank the influence of specific MPs by accounting for not only the number of speakers an MP influences, but also the importance of those influenced speakers.