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Gerrymandering, named after Founding Father and 5th U.S. Vice President Eldridge Gerry, is the process of creating legislative districts that benefit one party more than the other. Districts must be redrawn as a state’s population changes and because of shifts in population concentrations within a state. This process is often a political one because the very elected officials who have an electoral stake in the outcome of redistricting are often able to draw lines that benefit themselves and their co-partisans. Is there a way to keep politics out of drawing legislative districts?

What do I need to know about redistricting?

Article I Section II of the United States Constitution states that congressional districts should be drawn based on regular counts of the population. Today, this is done every ten years based on the new population numbers gathered in the U.S. Census.

All redistricting happens at the state level. Each state is allowed to set its own criteria for how district lines for congressional districts and state legislative districts are drawn. As a result, there is wide variation across the U.S. in how districts are drawn. Independent commissions, designed to take politics out of redistricting, were upheld as legal in the 2015 Supreme Court decision Arizona State Legislature v. Arizona Independent Redistricting Commission. As of 2023, nine states redistrict through independent commissions.

What is gerrymandering?

Map drawers begin with the results from the Census and group voters into districts following the guidelines above. The two most common methods of gerrymandering districts are to “crack” and “pack” constituents in districts to suit the goals of the map drawers. 

“Cracking” means dividing a given group and spreading them across several districts, thus reducing their voting power. “Packing” means trying to fit as many group members into as few districts as possible. One interesting result of partisan gerrymandering is that it can weaken a political party’s position and lower voter turnout. When districts are made less competitive, neither party leaders nor average voters are as likely to mobilize. 

One way to measure gerrymandering is using the “efficiency gap.” Efficiency gaps measure a party’s ‘wasted votes’ or votes cast for their losing candidates plus votes cast above what their winning candidates needed to win their elections. To calculate the efficiency gap, each party’s wasted votes in all districts are divided by the total number of votes they receive across all districts. An efficiency gap exists when the wasted vote ratio is higher for one party than the other. 

The existence of an efficiency gap suggests that districts are potentially gerrymandered. However, it does not prove gerrymandering. Even fair districting plans may create efficiency gaps because of how partisans are distributed throughout a state. For example, in most states, cities are more likely to have a majority of Democrats, while rural areas are more likely to have a majority of Republicans. The state of Georgia is one example, with Democratic voters concentrated in Atlanta. Because of this concentration, Georgia will almost always have safe Democratic seats in Atlanta and safe Republican seats in the surrounding rural areas. If districts follow the rules described earlier (keep communities of interest together, etc.,) there will be some efficiency gap regardless of how lines are drawn.

Maps drawn by commissions usually have smaller efficiency gaps and more competitive elections than maps drawn by state legislatures. By contrast, maps by state legislatures tend to have more safe districts and fewer competitive elections, reducing constituent electoral control and depressing turnout. 

How does gerrymandering work?

Here is an example of how gerrymandering works. Imagine a state where 60% of voters belong to the light gray party, 40% belong to the dark gray party, all voters vote for the candidate from their party, and the state elects five Representatives to Congress. However, the population is not evenly distributed – all of the dark-gray partisans live on the left-hand side of the state, and all of the light-gray partisans live on the right-hand side. The graphic below shows three different ways to draw district lines and the election outcomes they produce.

Source: Spence (2023)

In the left-hand panel, district lines are drawn north to south, and the number of seats each party gets is proportional to its number of partisans. Dark gray voters are “cracked” in the middle panel and split into all five districts. The result is that the light gray party wins all five seats by a 60-40 margin. In contrast, in the right-hand panel, light gray voters are “packed” into two districts, giving the dark gray three districts despite being only 40% of the population. 

The efficiency gaps in these three plans capture the amount of gerrymandering taking place. In the left-hand plan, the parties have an equal proportion of wasted votes. In the middle plan, the ratio is much higher for the dark gray party. In the right-hand plan, the wasted vote ratio is higher for the light gray party. 

What are the rules about redistricting?

Modern redistricting requirements began in the 1960s with a series of Supreme Court decisions. In 1962, the Court ruled in Baker v. Carr that states must re-evaluate and redraw district lines every ten years to account for population changes and movement. Before this ruling, many states had not redistricted in decades, even though the distribution of voters had shifted. In Tennessee, for example, one vote in a rural district was as powerful as 19 votes in an urban district.

The Carr decision (and subsequent cases) ruled that districts must be apportioned equally among the population. This is called the ‘one person, one vote’ principle – a vote in Congressional District 1 must have as much electoral power as a vote in Congressional District 2.

The manner in which states address race in redistricting has evolved. The Voting Rights Act of 1965 requires that everyone have equal access to voting, regardless of race, color, or language. 

The Supreme Court’s Shaw v. Reno (1990) decision held that redistricting plans based wholly on race violate the 14th Amendment’s Equal Protection Clause. This standard has been upheld as recently as 2023 with the case of Allen v. Milligan, when the Court rejected an Alabama redistricting plan that reduced the number of majority-black districts from two to one.

In addition to following the federal laws outlined above, there are three common legal standards. First, districts should preserve existing political subdivisions like county lines and townships. Second, districts should be contiguous (connected) with no “islands” (unconnected areas.) Third, districts should be geographically compact. Some states also ban changing lines in a way that would put two incumbents in the same district. Currently, nineteen states explicitly prohibit distributing plans that benefit one political party over the other. 

Proposed districting schemes are regularly challenged, on the basis of racial or population gerrymandering. In 2023, more than 70 lawsuits against congressional and state district maps challenged the legality of redistricting. While the Supreme Court has struck down maps based on racial discrimination, it has not done so for claims of partisan discrimination. In the 2013 decision of Gill v. Whitford, the Court sidestepped the matter, unanimously ruling it did not have constitutional authority on the case. 


Further Reading

Chen, J., & Rodden, J. (2013). Unintentional Gerrymandering. The Quarterly Journal of Political Science, 8(3), 239–269., accessed 2/21/24.

Crocker, R. (2012). Congressional Redistricting: An Overview. Congressional Research Service., accessed 2/21/24.

Stephanopoulos, N., & Warshaw, C. (2020). The Impact of Partisan Gerrymandering on Political Parties. Legislative Studies Quarterly, 45(4), 609-643., accessed 2/21/24.



What do I need to know about redistricting?

Crocker, R. (2012). Congressional Redistricting: An Overview. Congressional Research Service., accessed 2/21/24.

Engstrom, E. J. (2013). Partisan Gerrymandering and the Construction of American Democracy. Ann Arbor: Univ. Mich. Press., accessed 2/21/24.

National Conference of State Legislatures. (2021). Summary: Redistricting Criteria., accessed 2/21/24.

Nelson, M. (2023). Independent Redistricting Commissions Are Associated with More Competitive Elections. PS: Political Science and Politics 56(2), 207-212., accessed 2/21/24.

Ryan, J. M., & Lyons, J. (2014). The Effect of Redistricting Commissions on District Bipartisanship and Member Ideology. Journal of Elections, Public Opinion, and Parties. 25(2), 234-263., accessed 2/21/24.

What is gerrymandering?

Chen, J., & Rodden, J. (2013). Unintentional Gerrymandering: Political Geography and Electoral Bias in Legislatures. Quarterly Journal of Political Science, 8(3), 239–269., accessed 2/21/24.

Cox, G. W., Fiva, J. H., & Smith, D. M. (2020). Measuring the Competitiveness of Elections. Political Analysis, 28(2), 168-185., accessed 2/21/24.

DeFord, D. R., Eubank, N., & Rodden, J. (2022). Partisan Dislocation: A Precinct-Level Measure of Representation and Gerrymandering. Political Analysis, 30(3), 403-425., accessed 2/21/23.

McGhee, E. (2020). Partisan Gerrymandering and Political Science. Annual Review of Political Science, 23(1), 171-185., accessed 2/21/24.

Stephanopoulos, N., & Warshaw, C. (2019). The Impact of Partisan Gerrymandering on Political Parties. Legislative Studies Quarterly, 45(4), 609-643., accessed 2/21/24.

How does gerrymandering work?

Buzas, J. S., & Warrington, G. S. (2021). Simulated Packing and Cracking. Election Law Journal: Rules, Politics, and Policy, 20(4), 330-344., accessed 2/21/24.

Chen, J & Cottrell, D. (2016). Evaluating Partisan Gains from Congressional Gerrymandering: Using Computer Simulations to Estimate the Effect of Gerrymandering in the U.S. House. Electoral Studies, 44, 329-340., accessed 2/21/24.

Kenny, C. T., McCartan, C., Simko, T., Kuriwaki, S., & Imai, K. (2023). Widespread partisan gerrymandering mostly cancels out nationally, but reduces electoral competition. Proceedings of the National Academy of Sciences, 120(25)., accessed 2/21/24.

Spencer, D. (2023). Who draws the lines? All About Redistricting,, accessed 2/21/24. (Chart Data).

What are the rules about redistricting?

Brennan Center for Justice. (2023). Redistricting Litigation Roundup., accessed 2/21/24.

National Conference of State Legislatures. (2021). Redistricting and the Supreme Court: The Most Significant Cases., accessed 2/21/24.

Oyez. (n.d.). Allen v. Milligan, 599 US _. (2023)., accessed 2/21/24.

Whitaker, L. P. (2017). Congressional Redistricting Law: Background and Recent Court Rulings. Congressional Research Service., accessed 2/21/24.



Julia Acevedo (Intern) is a Political Science and Public Policy double major at Susquehanna University and is expected to graduate in May 2024 and pursue a Masters degree in Public Health.

Elijah Oaks (Intern) is a student at Dartmouth College. He is expected to graduate in May 2024 with a major in English and a minor in Religion. He is a Policy Fellow at The Cicero Institute.

Mary Adams (Team Lead) is a graduate student in the Department of Political Science at Indiana University and holds a Masters in Public Administration from Western Kentucky University. Her research is in American politics and political psychology.

Cory Colby (Subject Matter Expert) is Professor of Political Science at Lone Star College in Houston, Texas, where he is also the Civic Engagement Program Coordinator. He received an MA in Political Science from Sam Houston State University and was a congressional staffer for Representative Kevin Brady. His research focuses on electoral institutions and voter turnout.

Dr. Nathaniel Birkhead (Content Lead) received his PhD in Political Science from Indiana University. He is Associate Professor of Political Science and Department Chair at Kansas State University. His research focuses on American politics, especially Congress and state legislatures. 

Dr. William Bianco (Research Director) received his PhD in Political Science from the University of Rochester. He is Professor of Political Science and Director of the Indiana Political Analytics Workshop at Indiana University. His current research is on representation, political identities, and the politics of scientific research.

Published: 5/7/24

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