The two electoral arenas that co-habit mixed electoral systems interact, producing contamination effects. These raise the number of parties in the nominal arena above what Duverger’s Law suggests. However, the is a surprising assumption that contamination effects are an exclusively national phenomenon.

This study highlights the intersection of two important literatures: contamination effects in mixed-member systems and party system nationalization. It assumes that if national parties compete symmetrically in one district, but districts are heterogenous, then it is likely that they compete asymmetrically in other districts. In this sense, it assumes that the strength of contamination effects may vary in countries with heterogenous districts. Specifically, in countries with ethnicically dissimilar districts.

It relies on a large-N (cross-country) analysis that shows how contamination effects are higher in countries with low levels of party system nationalization, and that contamination effects are lower in countries with higher levels of party system nationalization.