Manipulating maps and winning elections: measuring the impact of malapportionment and gerrymandering

Geography is central to the operation of almost all electoral systems, through the interaction of two maps—the punctiform distribution of voters (and their political choices) and the territorial division of national space into constituencies. This interaction invariably results in election outcomes that are both disproportional and biased—with the allocation of seats being unequal to the distribution of votes across parties (and also across time for the same party). Such disproportionality and bias can be generated through the partisan strategies of malapportionment and gerrymandering, but in addition similar results can emerge when the procedure for defining constituencies is non-partisan. This paper argues that understanding the partisan and non-partisan nature of districting and its impact on election results requires formal measurement procedures and illustrates one such procedure, using the UK over the period 1950–1997 as its main exemplar.