Electoral Rule Disproportionality and Platform Polarization

This paper studies the effect of electoral rule disproportionality and the effect of the number of competing parties on the degree of platform polarization by the means of a unidimensional spatial model with policy motivated parties. We show that platform polarization is decreasing in the level of disproportionality of the electoral rule and increasing in the number of competing parties. By allowing parties to strategically decide whether to enter an electoral race or not we further show that the number of competing parties is decreasing in the level of disproportionality of the electoral rule. Therefore, an increase in the level of electoral rule disproportionality decreases platform polarization both directly and indirectly (through the reduction of the number of competing parties). By constructing a large and homogeneous dataset we provide empirical evidence in support of our theoretical findings; electoral rule disproportionality is the major determinant factor of the level of platform polarization while the number of competing parties has minor explanatory power (due to its dependency on the level of electoral rule disproportionality).