ABSTRACT
While conventional wisdom holds that partisan bias in U.S. legislative elections results from intentional partisan and racial gerrymandering, we demonstrate that substantial bias can also emerge from patterns of human geography. We show that in many states, Democrats are inefficiently concentrated in large cities and smaller industrial agglomerations such that they can expect to win fewer than 50% of the seats when they win 50% of the votes. To measure this ‘‘unintentional gerrymandering,’’ we use automated districting simulations based on precinct-level 2000 presidential election results in several states. Our results illustrate a strong relationship between the geographic concentration of Democratic voters and electoral bias favoring Republicans.

The very essence of districting is to produce a different -- a more "politically fair" -- result than would be reached with elections at large, in which the winning party would take 100% of the legislative seats. Politics and political considerations are inseparable from districting and apportionment. The political profile of a State, its party registration, and voting records are available precinct by precinct, ward by ward. These subdivisions may not be identical with census tracts, but, when overlaid on a census map, it requires no special genius to recognize the political consequences of drawing a district line along one street rather than another. It is not only obvious, but absolutely unavoidable, that the location and shape of districts may well determine the political complexion of the area. District lines are rarely neutral phenomena. They can well determine what district will be predominantly Democratic or predominantly Republican, or make a close race likely. Redistricting may pit incumbents against one another or make very difficult the election of the most experienced legislator. The reality is that districting inevitably has and is intended to have substantial political consequences.

It may be suggested that those who redistrict and reapportion should work with census, not political, data, and achieve population equality without regard for political impact. But this politically mindless approach may produce, whether intended or not, the most grossly gerrymandered results; and, in any event, it is most unlikely that the political impact of such a plan would remain undiscovered by the time it was proposed or adopted, in which event the results would be both known and, if not changed, intended.

It is much more plausible to assume that those who redistrict and reapportion work with both political and census data. Within the limits of the population equality standards of the Equal Protection Clause, they seek, through compromise or otherwise, to achieve the political or other ends of the State, its constituents, and its officeholders. What is done in so arranging for elections, or to achieve political ends or allocate political power, is not wholly exempt from judicial scrutiny under the Fourteenth Amendment. As we have indicated, for example, multi-member districts may be vulnerable, if racial or political groups have been fenced out of the political process and their voting strength invidiously minimized. See White v. Regester, supra; Whitcomb v. Chavis, supra. See also Gomillion v. Lightfoot, supra. Beyond this, we have not ventured far or attempted the impossible task of extirpating politics from what are the essentially political processes of the sovereign States. Even more plainly, judicial interest should be at its lowest ebb when a State purports fairly to allocate political power to the parties in accordance with their voting strength and, within quite tolerable limits, succeeds in doing so. There is no doubt that there may be other reapportionment plans for Connecticut that would have different political consequences, and that would also be constitutional. Perhaps any of appellees' plans would have fallen into this category, as would the court's, had it propounded one. But neither we nor the district courts have a constitutional warrant to invalidate a state plan, otherwise within tolerable population limits, because it undertakes not to minimize or eliminate the political strength of any group or party, but to recognize it and, through districting, provide a rough sort of proportional representation in the legislative halls of the State.