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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