Social Science Statistics Blog

The more recent literature on so called contamination effects undermines this claim. Several papers (Herron and Nishikawa 2001; Cox and Schoppa 2002; Ferrara, Herron, and Nishikawa 2005) have found evidence that there are interaction effects between the two tiers in mixed electoral systems. For example, small parties are able to attract more PR votes in those districts in which they run SMD candidates. The argument is that running a SMD candidate gives a human face to the party and thus enables it to attract additional PR votes.

In a recent paper, Holger Kern and I attempt to add to this debate by identifying incumbency as a source of contamination in mixed electoral systems. It is well known that incumbents that run in single-member district (SMD) races have a significant advantage compared to non-incumbents (Gelman and King 1990). It thus seems plausible to expect that this advantage carries over to the proportional representation (PR) tier, and that incumbents are able to attract additional PR votes for their party in the district. In our paper we identify such an effect using a regression-discontinuity design that exploits the local random assignment to incumbency in close district races (based on an earlier paper by Lee 2006). The RD design allows us to separate a subpopulation of district races in which treatment is assigned as good as randomly from the rest of the data that is tainted by selection effects. We find that incumbency causes a gain of 1 to 1.5 percentage points in PR vote share. We also present simulations of Bundestag seat distributions, demonstrating that contamination effects caused by incumbency have been sufficiently large to trigger significant shifts in parliamentary majorities.

Needless to say, any feedback is highly appreciated.