Structural constraints on partisan bias under the efficient gerrymander

Partisan bias occurs when the translation of the popular vote into legislat ive seats differs between competing parties. This paper contains a theoreti cal 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 envir onment; namely, the size of the voting population and the number of single- member districts within a political jurisdiction. A statistical analysis re veals the predicted relationships in data on Congressional elections across states in the 1950-1984 period. This paper highlights the importance of so me measurable features of the electoral environment for determining bias an d provides an indirect test of partisan gerrymandering in congressional app ortionment processes.