Ludovic Rheault, André Blais, John H. Aldrich, Thomas Gschwend
Understanding people's choice when they have two votes

Journal of Elections, Public Opinion and Parties, 2020: 30, issue 4, pp. 466-483
ISSN: 1745-7289 (print), 1745-7297 (online)

This paper introduces a model of vote choice in mixed-member proportional representation systems where electors cast two votes. Despite the growing popularity of mixed systems around the world, a recent stream of literature suggests that the candidate vote contaminates the list vote, inducing the type of behavior observed under majority rule. We propose a new approach to account for these so-called “contamination” effects, a phenomenon that we define as a causal influence making choices more similar across the vote decisions. Since causality entails a time ordering, we argue that contamination arises only when voters choose sequentially. By making use of new survey questions asking respondents about the timing of vote decisions, we can estimate the magnitude of these contamination effects directly. The model is tested using Bayesian multinomial probit models with survey data from the 2013 federal election in Germany. A key contribution of this paper is to show that contamination effects are present only among voters with lower levels of education, and work primarily from the list vote to the candidate vote. We also test a number of predictions about the determinants of the two vote choices in mixed systems.