Despite common perception, existing theoretical literature lacks a complete for-
mal argument regarding the relationship between the electoral rule disproportionality and platform polarization. In this paper, we build a model that incorporates the disproportionality of the electoral system in a standard Downsian electoral competition setup with mainly, but not necessarily purely, policy-motivated parties.
We first show that in equilibrium, platform polarization is decreasing in the level
of the electoral rule disproportionality. We then argue that the number of parties
has a positive effect on platform polarization when polarization is measured by
the distance between the two most distant platforms. This effect does not hold
when polarization is measured by the widely used Dalton index. Constructing a
dataset covering more than 300 elections, our main theoretical findings are empirically supported, pointing towards the electoral rule disproportionality as a major determinant of polarization.