01094pab a2200145 454500008004000000100002000040245007800060260000900138300001300147362000800160520070200168650002200870650002200892773003400914180718b1999   xxu||||| |||| 00| 0 eng d  aTaagepera, Rein  aThe number of parties as a function of heterogeneity and electoral system  c1999  ap.531-48  aAug  aWhat determines the number of parties in a national assembly? Previous work has emphasized either sociopolitical heterogeneity or electoral system permissiveness, or brought them together on an empirical basis. Here an equation is developed that satisfies two theoretical boundary conditions and expresses the effective number of assembly parties (N) in terms of both the number of politicized issue dimensions (I) and effective magnitude (M) of electoral system: N=I6M15+1. Actually, depending on circumstances, any of the three variables could become the dependent one, affected by the two others. Empirical evidence is presented, based on Lijphart's (1984) data on 22 stable regimes. Reproduced  aElectoral systems  aPolitical parties  aComparative Political Studies