Party systems are complex and the literature consequently employs many different dependent variables to asses its development. They include the formation of voter preferences, the partisan alignments of such preferences, the volatility of such alignments, the formation and turnover of governing coalitions and electoral fragmentation. From this long list, electoral fragmentation – that is the number of parties and their respective electoral strength (i.e. effective number of electoral parties, ENEP) or parliamentary strength (i.e. effective number of parliamentary parties, ENPP) – is the most widely used measure of party system formation and, hence, the one that I use. (Taagepera and Shugart 1989, 77-91) The literature uses fragmentation to infer when a party system reaches a stable equilibrium point and hence can be considered institutionalized. According to Gary Cox, a stable equilibrium corresponds to a set of market clearing expectations where “the number of types of candidates that voters are willing to vote for turns out to equal the number and types of candidates that are willing and able to stand for elections.” (1997, 7) Equilibria centrally form around mutual expectations that voters and politicians have of each other, and they are reached when neither politicians nor voters have any incentive to change their behavior.
What specifically forms and coordinates the expectations of actors is widely debated. Figure 1 groups the most commonly cited factors into three broad causal categories. The most common explanations focus on the three bold boxes 2, 4, and 5 and identify institutions and strategic voting as the principle factors inducing equilibria. Two less common explanations emphasize historical factors (dotted box 1) and political strategies. (dotted box 3) Each approach identifies a factor that seems important in shaping party systems, yet none of them pays much attention to the two others. I briefly discuss each approach and identify certain problems that result from their insufficient dialogue.