What is Gerry?
Gerry is a redistricting simulator that demonstrates how drawing district boundaries can determine election outcomes independently of how people vote. Players partition a procedural hex-tile map of voters into districts and immediately see how their choices affect six fairness metrics.
Redistricting Mechanics
The map generates voters affiliated with three political parties, distributed across population centers with realistic urban-suburban-rural gradients. Players paint hexes to assign them to districts. Each district elects one representative by plurality vote. Real-time feedback shows vote shares, seat counts, and all six fairness metrics as boundaries change.
Fairness Metrics
Six metrics evaluate every map: the efficiency gap (all-party wasted votes as a fraction of total), partisan symmetry (pairwise vote swaps across all three parties), competitive district count (margins under 10%), compactness via the Polsby-Popper ratio, contiguity (no isolated fragments), and majority-minority district count. No single metric captures fairness completely — the simulation shows how they can conflict.
Algorithms
Two automated modes contrast deliberate gerrymandering with a neutral geometric draw. Pack-and-crack concentrates opposition voters into a few districts while spreading the rest thin. Fair draw runs fifteen Lloyd-style Voronoi relaxation rounds, assigning cells with a distance-priority multi-source flood fill whose soft population penalty discourages overfilled districts. Monte Carlo elections stress-test any map with 50, 100, or 500 trials using correlated national party swings plus smaller district-level preference noise.
Voting Rights Context
In the United States, the Voting Rights Act of 1965 prohibits discriminatory vote dilution. Gerry tracks districts where its generated minority population exceeds half the district population and compares the count with a deliberately simplified statewide-share heuristic. The simulator does not test geographic compactness, communities of interest, racially polarized voting, or the other facts required for a legal analysis.
Accessibility
Gerry's buttons, native controls, sidebar tabs, district selectors, and documented shortcuts are keyboard-operable. The map itself exposes an accessible label, but assigning individual hexes still requires a pointer or touch; this is a known gap. Toolbar actions have ARIA labels, and district results are reported as text and numbers alongside the color-coded map. The theme toggle switches between light and dark presentations. Map generation and painting use brief transitions; the shared stylesheet collapses them when the operating system requests reduced motion.
See also: Geon for particle physics, Shoals for options trading, Cyano for cellular metabolism.
Learning Outcomes
After using Gerry, students should be able to: explain how pack-and-crack strategies manufacture electoral majorities from minority vote shares; compute the efficiency gap for a given district map and interpret the simulator's 7% warning threshold; describe the trade-off between compactness and proportional representation; and evaluate why contiguity alone is insufficient to guarantee fair districts.
Prerequisites
No prerequisites beyond basic arithmetic. The simulator introduces all fairness metrics with visual feedback — no prior knowledge of electoral law or political science is needed.
References
N. Stephanopoulos and E. McGhee, "Partisan Gerrymandering and the Efficiency Gap" (2015). D. Polsby and R. Popper, "The Third Criterion: Compactness as a Procedural Safeguard Against Partisan Gerrymandering" (1991). Rucho v. Common Cause, 588 U.S. ___ (2019).