The Cube Rule in a Mixed Electoral System: Disproportionality in German Bundestag Elections

In the German mixed electoral system the PR tier is generally perceived as fully compensating for any disproportionality in the vote–seat translation generated by the plurality tier. However, as this article shows, the PR tier can itself increase disproportionality. In a mixed electoral system, small parties enter (hopeless) district races with the hope of boosting their PR vote share. But with a high number of district parties, parties may win districts at levels way below the usual 50 per cent vote share threshold. Looking at all 16 Bundestag elections from 1953 to 2009, the article identifies the effective number of district parties as a very strong predictor for the disproportional translation of votes into seats in the plurality tier of Germany’s mixed electoral system. The article points to consequences for the internal composition of parliamentary parties, for parties’ nomination strategies and for the occurrence of so-called overhang mandates.