Evaluating Partisan Gerrymandering in Wisconsin

TL;DR

This paper analyzes the extent and impact of partisan gerrymandering in Wisconsin's 2010 district map, revealing significant bias that favors Republicans and affects election outcomes.

Contribution

It provides a comprehensive evaluation of gerrymandering's effects and demonstrates Wisconsin's district plan as an outlier with significant partisan bias.

Findings

Wisconsin's district map is highly gerrymandered.

The map skews results in favor of Republicans when Democratic votes exceed 50%.

Gerrymandering likely altered the partisan composition of the Assembly.

Abstract

We examine the extent of gerrymandering for the 2010 General Assembly district map of Wisconsin. We find that there is substantial variability in the election outcome depending on what maps are used. We also found robust evidence that the district maps are highly gerrymandered and that this gerrymandering likely altered the partisan make up of the Wisconsin General Assembly in some elections. Compared to the distribution of possible redistricting plans for the General Assembly, Wisconsin's chosen plan is an outlier in that it yields results that are highly skewed to the Republicans when the statewide proportion of Democratic votes comprises more than 50-52% of the overall vote (with the precise threshold depending on the election considered). Wisconsin's plan acts to preserve the Republican majority by providing extra Republican seats even when the Democratic vote increases into the range when the balance of power would shift for the vast majority of redistricting plans.