In Chapter 1, we suggested a number of ways that we believe electoral rules may affect politics while also being conditioned by other factors. In this chapter, we discuss how studying mixed-member electoral systems with an approach that Arend Lijphart labeled “controlled comparison” can help us investigate these electoral rule effects.

In the first part of this chapter, we describe the basic workings of “two-vote” mixed-member systems and explain how these systems make the controlled comparison approach possible. We highlight how this approach to the study of mixed-member systems offers a number of advantages in studying the differing effects of SMD and PR electoral rules. In the second part of the chapter, we discuss “contamination” between the PR and SMD tiers – the chief misgiving offered by critics of the controlled comparison approach to studying mixed-member systems – and explain why we believe that contamination does not cause significant problems for the analysis in this book. In the third and final part of the chapter, we describe the defining characteristics of mixed-member systems, outline the major variations found within this classification of electoral system, and discuss how these variations may have important consequences of their own.