What 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 = I.6M.15 + 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.
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