Most past research on legislative voting behaviour in parliamentary systems is primarily concerned with explaining variation in aggregate-level scores of party unity (at parliament or party level), hence overlooking the agent at the core of legislative vote choice – the individual MP. Picking up on this analytical gap, the paper is guided by the question as to why some MPs display a more loyal voting behaviour than others in respect to the party line. The innovative theoretical focus moves beyond the prevalent assumption that MPs chiefly seek to influence policy making and extends to MPs’ socio-economic motivations. By developing a model of dependence between the MP (seeking career goods) and the parliamentary party group (PPG) (seeking party unity), the paper explores the potential effect of MPs’ career backgrounds on their voting behaviour. The testable logic assumes that MPs with extra-parliamentary careers are more immune to PPG threats and rewards issued in response to MPs’ voting behaviour; these legislators possess plausible career alternatives as exit-options, lowering their transaction costs in case of non-renomination or denial of parliamentary/PPG career advancement. This increased independence is expected to manifest itself in a lower degree of party loyalty in legislative voting. Empirically, these claims are tested for two legislative periods in the German Bundestag (2005-2009, 2009-2012). The quantitative analysis draws on an original large-N (150,000+ observations) dataset and employs multilevel modelling to account for the disaggregated data structure and dependencies between observations. While controlling for previous findings and alternative explanations, it is shown that the odds of MPs with external careers voting against the party line are significantly higher than the odds of MPs without outside careers doing so. With its unique approach and findings, the paper aims to advance the under-developed field of micro-level analysis of party unity and emphasises the need for further research.
