Distinguishing between the effects of swing ratio and bias on outcomes in the US Electoral College, 1900–1992

Abstract

We review a number of different statistical techniques for creating seats-votes curves and apply the most reliable of these to estimate seats-votes relationships in the US electoral college 1900–1992. We consider the now rejected claim, once firmly established as part of the common journalistic and even academic wisdom, that the US Electoral College has recently been strongly biased in favor of Republicans, and show that this claim was based largely on a confusion between bias ( asymmetry in the electoral college gains earned by the votes received by different parties or candidates) and swing ratio ( responsiveness of change in electoral college seat share to change in popular vote). Although there has been substantial bias during this century in the way the electoral college translates Democratic and Republican votes into electoral college seats, and for the earlier party of this century (from 1900 to 1940) that bias has been in favor of Republicans, to explain why many recent electoral college majorities have been so lopsided we must look not at bias but at swing ratio. We show that the swing ratio in the electoral college has generally been increasing since 1900, rising from an average value (1900–1924) around three to an average value (1976–1992) ranging from about five to about eight, depending upon which of the various statistical estimation techniques we use. Thus, for every one point vote share gain above 50 per cent, a winning presidential candidate in a two-candidate competition can now expect to pick up somewhere between a 5 percentage point and an 8 percentage point increase in electoral college seats—giving the illusion of mandate even for relatively close contests and frequently creating apparent landslides. We show that this historical rise in swing ratio in presidential elections is due almost entirely to changes in the responsiveness of outcomes in the US South as the influence of the Civil War slowly (very slowly) erodes. Drawing on the analysis of the determinants of bias and of swing ratio in the House of Representatives in Brady and Grofman (1991b), we show that the increases in electoral college swing can be accounted for by the nationalization of presidential competition as signaled by the decrease over time in the standard deviation of Democratic share of the two-party vote across states, and that changes in bias can be linked to changes in the magnitude of differences between the mean and the median of that distribution.