A Tradition as Old as the Republic

Gerrymandering, or the practice of manipulating electoral district boundaries for political gain, is a practice as old as America itself.

Where the lines are drawn affects both state and federal elections. For Congressional elections, states are divided into districts based on how many representatives they send to the House of Representatives. In these cases, partisan gerrymandering is often used to maximize the wins for their own party.

Govenor Gerry
Massachusetts Governor Gerry first redrew the map of districts in an unusual way when he took office in 1810 to weaken the Federalist Party. Newspapers ridiculed the salamander-shaped districts, calling them "Gerrymanders", a term that stuck.


How does gerrymandering impact elections?

Gerrymandering often occurs to secure party control, favor incumbents, or dilute the voting power of certain demographic groups. There is a fine line between an appropriate amount of community representation and partisan favoritism. The ambiguity of the situation is exacerbated by the lack of standard measurements of fairness.

Imagine a state of 50 people where 60% are blue and 40% are red. Is there a way that reds could win, despite being a minority?

Visualization adapted from Stephen Nass's "How to steal an election"

Windiest Districts in America

Many of the resulting districts end up as strange, winding shapes that intentionally include or exclude populations for political gain.

As a result, even though Party A may win the popular vote, Party B may end up sending more representatives to congress. Whoever decides the lines decide whose vote is reflected. This can skew statewide representation, dilute minority votes, split communities.

The Art & Science of Map Manipulation

The goal of gerrymandering is to maximize the partisan number of districts that will vote for your party. This partisan distribution requires balancing the risk of maintaining a comfortable lead with the need of minimizing vote wastage.


Place all of your opponent's supporters into a few foregone districts to maximize wasted votes


Distribute your own supporters evenly amongst the districts to ensure a comfortable majority

Who draws the lines?

District lines are redrawn every 10 years to reflect the updated populations taken by the US Census.

In most states, district lines are drawn by the majority party of the state legislature and approved by governor. Even in states with alternative processes like Independent Commissions, there are seats specifically reserved for Democratic or Republican appointees, so there are no truly non-partisan redistricting committees.

Visualization adapted from All About Redistricting

Tracing the legal and cultural history of gerrymandering

Click on an event to learn more

A Rule for Fairness

To measure "fairness", numerous metrics have been proposed, including a measure of compactness (the ratio of perimeter to area, with the ideal being a perfect circle at 1) and more recently, the efficiency gap, which measures the number of both wasted vote and diluted votes.

The efficiency gap measures net wasted votes by both parties over the total votes. The metric is compared against the number of seats won to find any seat advantages attained by either party.

The Gill v. Whitford case heard by the Supreme Court in October 2017 offers an opportunity to set a precedent for a legal test of fairness.

Calculating the Efficiency Gap

Seat margin - (2 * vote margin)

Seat margin: share of seats won by the winning party minus 50%

Vote margin: share of votes won by the winning party minus 50%

For uncontested races, we assumed that the opponent would have won 25% of the votes, as both the Associated Press and the University of Chicago Law School utilized this 75-to-25 vote share assumption in their models.

How Gerrymandered is your district?

Mouse over to explore the 2014 elections (latest complete available election data set):

Select District

Efficiency Gap:

Election Outcome:

Contested Seats:

Alternatives for Drawing Districts

Independent redistricting commissions

Canada shifted to this model in 1960s, but in the US, politicians still name appointees to the district.

Algorithm-based districting

Algorithms group on compactness and grouping an equal distribution of voters, but have a difficult time factoring in shared communities of interest.

Proportional representation

Allocates representatives proportionally to voting results across the state instead of the current Winner Take All model, which uniquely solves judicial meddling and natural geographic clustering.

While Proportional Representation would most fairly represent a state's interests, it also requires the most drastic changes to the current politican system. For now, the outcome of the Supreme Court's decision for Gill v. Whitford will potentially provide a precedent for states to test the fairness of districting as a single, universal standard.

In light of the impending 2020 Census redistricting, the shape of these districts could dramatically impact the 2020 Presidential Election and the future of America.

“ If you can stack a legislature in this way, what incentive is there for a voter to exercise his vote? Whether it’s a Democratic district or a Republican district, the result is preordained in most of the districts[...] What becomes of the precious right to vote?”

— Ruth Bader Ginsburg

Further Reading