Table of Contents Introduction I. Background A. Current Legal Constraints on How a Partisan Gerrymander May Be Defined B. Searching for a Manageable Standard: The Current State of Play C. Mathematical Methods Can Identify National- and State-Level Imbalances II. Quantitatively Analyzing the Effects and Intents of Partisan Gerrymandering A. Analysis of Effects: What Is an Appropriate Range of Seats for a Given Share of Votes? 1. Distinguishing partisan distortion from Voting Rights Act section 2 constraints 2. Defining the zone of chance 3. National districting patterns can be used to identify a natural seats/votes relationship 4. What accounted for the antimajoritarian outcome of 2012? B. Analysis of Intents: Voter Packing by Intentional Gerrymandering and Self-Association 1. Distinctive patterns of win and loss margins arising from partisan gerrymandering and voter self-association 2. Gerrymandering emulates and amplifies the representational consequences of urbanization 3. A "lopsided-margins test" to detect when the targeted party wins with unusually large margins 4. The mean-median difference as a measure of skewness 5. State-by-state comparisons of skewness with population clustering effects III. Three Quantitative Tests for the Detection of the Effects of Partisan Gerrymandering A. Converting the Analyses to Practical Tests B. Advantages and Disadvantages of the Three Tests C. Three Examples: The Original Gerry-mander, Arizona State Legislative Districts, and Maryland Congressional Districts IV. Discussion A. Allowing for Ambiguity B. What Is the Role of Intent? C. Evaluating the Partisan Impact of District Maps Before Implementation Conclusion Introduction
Partisan gerrymandering, in which geographical jurisdictions are divided to give special advantage to one political group over others, is quite old, dating to the establishment of Pennsylvania's assembly districts in 1705. (1) The term "Gerry-mander" was later coined in 1812 to mock an oddly shaped district encompassing northern parts of Essex County, Massachusetts. (2) The broader target of editorial scorn, however, was the overall goal of gaining more seats at the statewide level than the party's support among the population would normally justify. For the "Gerry-mander," redistricters from Massachusetts--specifically, Governor Elbridge Gerry's Democratic-Republican Party--sought to take popular support that was closely divided between their party and the other major party, the Federalists, and divide it among districts to favor their own side. (3) The stratagem worked: Federalists won the two-party vote share by a margin of 51%-to-49% over the Democratic-Republicans, but ended up severely outnumbered in the General Court, with only eleven seats to the Democratic-Republicans' twenty-nine seats. (4) Federalist voters were packed so that Federalist candidates won an average of 71%-to-29% of the two-party vote in the districts they carried. (5) Democratic-Republicans were distributed to allow wins in a larger number of districts, averaging 56%-to-44% per district. (6) This result exemplifies a central principle of partisan gerrymandering: concentrate voters on a district-by-district basis such that both sides' wins are reliable, but the redistricting party's victory margins are smaller than those of the opposing party and are thereby used more efficiently.
The seat advantage gained in a partisan gerrymander represents a distortion arising from the districting process that causes election results to deviate from natural patterns. Such distortions, however, do not necessarily persist over time. In the case of the original "Gerry-mander," the next election, in 1813, showed a rapid reversal of fortune for the Democratic-Republicans. (7) Public anger over the War of 1812 and over gerrymandering itself (8) led to increased Federalist turnout, and a 56%-to-44% popular-vote victory, with an outcome of twenty-nine Senate seats to the Democratic-Republicans' eleven. (9) This perfect reversal of outcomes was achieved with only a five percent increase in the Federalists' vote share. Such a dramatic swing was possible because Democratic-Republican-leaning districts were engineered to deliver extremely narrow victories, so that a small swing in opinion was sufficient to influence many races.
The example of Massachusetts in 1812 and 1813 shows that a partisan gerrymander's effects can be reversed if voter sentiments change sufficiently. A gerrymander can also weaken if voters physically change residence. When district boundaries are carefully constructed based on the pattern of voter residence at a single point in time, it is more likely than not that voter mobility will tend to dissipate the advantage, much as a child's carefully built sandcastle, once left unattended, will erode with the wind.
Finer-grained drawing of boundaries and technological advances have since opened the possibility of drawing more sophisticated gerrymanders that potentially lead to more secure and lasting advantages for the party in charge of redistricting. Several factors come into play.
First, redistricting was once done on a county-by-county basis. (10) Detailed census and voter-registration information is now available, allowing redistricters to construct districts on a block-by-block basis. (11) Districting software, in both commercial and freely available varieties, allows users to access this information to explore many scenarios in rapid succession and to create boundaries that separate different populations of voters in exquisite details. Professionals use proprietary software to draw districts, but even activists and ordinary citizens can enter the fray using free software such as Dave's Redistricting App. (12)
Second, voters themselves have tended to cluster into Democratic- and Republican-preferring communities. Generally speaking, Democratic voters are found more often in regions of higher population density, and Republican voters in regions of lower population density. These tendencies have intensified in recent years as part of a phenomenon termed "the Big Sort." (13) This sorting leads voters to become self-aggregated into easy-to-handle contiguous chunks, within which partisan preference is strong in one direction or the other. Overall, reliable partisan voting and the Big Sort create geographic patterns that make it easier to gerrymander. In this way, polarization can facilitate gerrymandering. (14) Furthermore, safely held seats, whether they arise from polarized communities or from gerrymandering, insulate representatives from voter preference.
Based on analysis in the 1990s, the effects of partisan congressional gerrymanders have been estimated to last for multiple election cycles, but with the potential to diminish after even one election cycle. (15) The Big Sort may allow redistricting to have longer-lasting effects as neighborhood-level partisan tendencies become more stable. In addition, changes in technical tools and population clustering, as well as a greater awareness of the advantages of aggressive districting, further enhance the possibility that gerrymandered districts may be more durable now than they were even ten years ago. (16)
Often, a two-party system exhibits a high degree of partisan symmetry: if the major parties were to switch vote share, they would also come close to switching their share of seats in the legislative body. However, partisan gerrymandering has reached recent extremes of asymmetry as an increasing number of state governments have come under one-party rule. (17) All these factors working together--the Big Sort, more detailed data, computer-based districting, and single-party rule--provide easier routes to give undue advantage to whichever political party controls redistricting. These factors magnify the need for a manageable standard to define--and potentially curb--partisan gerrymandering.
In this Article, I present three tests that address the problem of detecting extreme deviations from partisan symmetry. First, in Part I, I review court precedents that establish the desirability of partisan symmetry as an outcome, a concept that can be used to help define a partisan gerrymander. In Part II, I describe mathematical approaches, grounded in longstanding statistical practice, to detect partisan asymmetry. I present two analyses: one that measures effect, which I define as the number of seats that are gained by a gerrymander, and one that detects intent, which I define as a pattern of district-level partisan outcomes that is unlikely to have arisen by chance. The number-of-seats measure specifically overcomes the central difficulty that representation is not necessarily proportional to public support. Nonproportionality has long been known to arise naturally from the winner-take-all nature of individual elections. (18) My calculation of effects replaces the intuitive, but incorrect, ideal of proportionality.
In Part III, I use these analyses to construct three tests to evaluate cases of gerrymandering. I apply my tests to example cases, starting with the original Gerry-mander of 1812, up to post-2010 congressional districting plans in all fifty states. Further, I also consider two recent cases that have come before the Supreme Court: the Maryland congressional delegation, in Shapiro v. McManus, (19) and the Arizona state legislative districts, in Harris v. Arizona Independent Redistricting Commission. (20) The results of the tests support the idea that gerrymandering has distorted the composition of Maryland's congressional delegation and has made it unresponsive to changes in voter sentiment. By contrast, Arizona legislative districts do not show significant asymmetry. In Part IV, I conclude by suggesting ways in which these tests can be used to construct a manageable standard for use by courts and legislatures. These tests are available for online use at...