How (Not) to Tell You’ve Been Gerrymandered (Part I of Several)

In a previous post, we explained how sophisticated map analysis can support drawing district lines that result in party representation in the legislature that is dramatically different from the distribution of support for a party in the population. Over several upcoming posts, we will describe the analyses that are being used to identify biased district maps and to try to persuade courts of bias. The analyses are somewhat mathematical but largely not beyond the comprehension of the general reader with modest effort. In this outing, we provide some background on the court decisions that set the parameters for these analyses.

What does the Constitution of the United States of America require regarding district maps? First, that a census be conducted every ten years and that representatives be apportioned to the states according to their populations, except that every state gets at least one representative. The Supreme Court of the United States has ruled that all districts that elect representatives to any office except the United States Senate must have equal-sized populations. This is the principle known as “one person, one vote.”

Section 4 of the Constitution states:

The Times, Places and Manner of holding Elections for Senators and Representatives, shall be prescribed in each State by the Legislature thereof; but the Congress may at any time by Law make or alter such Regulations, except as to the Places of chusing Senators.

In the absence of Federal legislation to thwart partisan gerrymandering, some politicians and strategists argue that the power to draw maps rightfully belongs to the legislature and that there is nothing wrong with the majority party controlling the outcome of that process. Opponents of partisan gerrymandering argue that, as a consequence of the First Amendment’s guarantee of freedom of assembly and the Fourteenth Amendment’s guarantee of equal protection of the laws, that maps with excessive partisan bias should be considered unconstitutional. The question has come before the United States Supreme Court several times, but has not been ruled on decisively.

In the 2004 case, Vieth v. Jubelirer, the Court considered whether the question of partisan gerrymandering was even justiciable. The Court split four-four on the question. Justice Kennedy sided with the conservatives in declining to intervene in the case, but whereas the four conservative justices argued that partisan questions could not be justiciable, Kennedy wrote a separate opinion arguing that if appropriate standards could be developed, partisan gerrymandering could be taken up in the future. It is Kennedy’s opinion that has spurred efforts to develop the analyses that are being used in current cases.

One simple idea for a test is proportionality, the idea that the number of seats a party holds should approximately reflect the party’s support in the general population. Indeed, stories in the press often note violations of proportionality as an indicator of bias and it can serve as a red flag, prompting further analysis. But proportionality has several problems that make it unsuitable as a standard for evaluating partisan bias.

An important mathematical issue is that when voters’ party preferences are not extreme, the seat balance is sensitive to small differences in party preference. In particular, on average, a specified level of voter preference for the majority party tends to translate to an even higher percentage of seats. In addition, courts are reluctant to consider proportionality as a criterion, in part because it is difficult to establish standing to sue. In the U.S. legal system, standing is achieved by demonstrating that one has suffered direct harm, which is difficult to argue based solely on lack of proportionality.

In future posts, we will describe the methods that are currently being used in gerrymandering court cases and in support of legislative initiatives.

Published by Matthew Saltzman, Ph.D.

Associate Professor of Mathematical and Statistical Sciences, Clemson University. President, the COIN-OR Foundation, Inc. Director for Redistricting, League of Women Voters of South Carolina

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