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COMPUTING SCIENCE

Machine Politics

Brian Hayes

The Good District

There is no consensus on what qualities a good redistricting plan ought to have, but here are some widely recognized desiderata:

  • All the districts within a state should be equal in population. This is the burden of the "one person, one vote" rule enunciated in a series of Supreme Court decisions beginning with Baker v. Carr in 1962. The court has enforced a remarkably stringent standard of numerical exactness; in 1983 a New Jersey plan was struck down for numerical inequities of 0.69 percent. (For comparison, the error in the 1990 census is estimated at 1.4 percent, and from state to state the average population of a Congressional district varies by almost 60 percent.)
  • Each district should be a single contiguous territory. At least some states accept the minimal definition of contiguity, allowing regions connected by a single point. There are two places in North Carolina where districts cross each other, which implies that the connecting isthmus must be a dimensionless point.
  • Districts should be compact. Tentacles wriggling across the landscape arouse suspicions, including those of Supreme Court justices. (The North Carolina 12th district is 165 miles long but so narrow that a candidate for the seat remarked, "I can drive down Interstate 85 with both car doors open and hit every person in the district.") Compactness is surprisingly tricky to define and measure. Computational geometry might seem to offer guidance here, but H. Peyton Young of the University of Maryland has shown that various mathematical measures of compactness yield counterintuitive results. For example, a spiral tract of land that winds around itself like a coiled snake passes several tests of compactness, but few would consider it a natural shape for a Congressional district.
  • Districts should recognize existing communities of interest. This is an argument for homogeneity, for creating districts that are uniformly urban or rural, coastal or mountainous, industrial or agrarian, etc.
  • Similarly, districts should conform to established natural and political boundaries whenever possible. Until 1990 North Carolina districts were always assembled from whole counties, but that practice has had to yield to other imperatives.
  • Stability and continuity are virtues in a redistricting plan. A procedure that starts from scratch and draws an entirely new map every decade is likely to be unpopular not only with incumbents but also with constituents.
  • Finally, under the Voting Rights Act a district must not be drawn with the intent or the effect of excluding minority candidates from election.

Conflicts between these criteria are commonplace. Creating minority-majority districts may require splitting counties; and the one-person, one-vote rule can put all the other factors in jeopardy. When compromise is necessary, the courts have given the highest precedence to numerical equality.





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