Abstract: For a given electoral system, what average number and size distribution of parties can we expect? This book makes specific predictions that agree with world averages. The basic factors are assembly size and district magnitude (the number of seats allocated in the district). While previous models tell us only the direction in which to change the electoral system, the present ones also tell us by how much they must be changed so as to obtain the desired change in party system and cabinet duration. These are quantitatively predictive logical models. Combined with known particularities of a country, these models can be used for informed institutional design. Allocation of seats among countries in the European Parliament is also put on a logical basis.

Keywords: electoral systems, party systems, number of parties, size distribution, cabinet duration, quantitatively predictive models, assembly size, district magnitude, institutional design, seat allocation