The importance of electoral rules: Comparing the number of parties in Spain's Lower and Upper Houses

In this paper we study a coordination dilemma that appears in democracies where the Upper and Lower Houses are elected simultaneously using significantly different rules. How do party systems form when some parties are only viable in some districts of one of the chambers? Are party systems independent from one another, as Duvergerian theories expect, or dependent, as the literature on the interactive features of mixed electoral systems predict? Using the case of Spain, where all the possible incentive structures for electoral coordination are present, we test both theories. Our results point out that contamination effects matter, but they are not strong enough to make Duvergerian gravity disappear. Both effects are at play in Spanish elections.