Contextual Determinants of Electoral System Choice - Ã bo Akademi

attention not only to the number of parties but also to their relative strength, i.e. thevote (or seat) shares for each party. In 1967, Rae constructed an index of partysystem fractionalization, ranging from 0 to 1. Low values indicate a low degree ofparty system fragmentation and vice versa. Rae’s index has later on been modified.Perhaps the most widely used measure of party system fragmentation is ‘theeffective number of parties’, created by Markku Laakso and Taagepera (1979). Theeffective number of parties is calculated according to the following formula:1N = -------∑vі²where vі refers to the vote share of the i th party.The smallest possible value is 1. No upper limit exists. The effective number ofparties can be calculated on the basis of either vote shares or seat shares. In thiscase, vote shares are more appropriate. If the party system is assumed to affect thechoice of electoral system, the electoral support reflected in the vote share of eachparty is the crucial factor. If the effective number of parties is calculated on thebasis of seat shares, each value is influenced by the electoral system used in theprevious elections, because the electoral rules determine the allocation of seats. Inthe theoretical survey of party systems and electoral systems, it was argued thatboth variables affect each other. A fragmented party system might be theconsequence of a proportional electoral system as well as the initial reason forchoosing that kind of system. The application of vote shares rather than seat sharesdecreases the influence of the former electoral system on the party system structureto a considerable extent. Notwithstanding, the allocation of vote shares is nottotally independent of the electoral system, because the electoral system also has apsychological effect, in addition to a mechanical effect, on the party systemstructure. 25To use the effective number of parties as a measure of party system structureimplies that I must exclusively focus on cases of electoral system change. Even if arobust measure of party system fragmentation would be applied, the estimation ofthe number of parties before the choice of electoral system and the first electionswould be rather vague. By regarding the effective number of parties in countries25 See e.g. Duverger (1964) for a description of the psychological and the mechanical effect of electoral systems onparty systems.122