Structural constraints on partisan bias under the efficient gerrymander

Partisan bias occurs when the translation of the popular vote into legislative seats differs between competing parties. This paper contains a theoretical and empirical analysis of the consequences of an efficient gerrymander for the partisan bias of an electoral system. Under partisan apportionment, bias is shown to depend on some structural features of the electoral environment; namely, the size of the voting population and the number of singlemember districts within a political jurisdiction. A statistical analysis reveals the predicted relationships in data on Congressional elections across states in the 1950–1984 period. This paper highlights the importance of some measurable features of the electoral environment for determining bias and provides an indirect test of partisan gerrymandering in congressional apportionment processes.