Is partisan gerrymandering widespread in the state legislatures? We assess state legislative redistricting maps approved by state governments in 2011. We find results that are similar to estimates of districting bias in the US House. On average, partisan bias increased after redistricting. State governments approved more than forty state legislative redistricting plans that gave one party an extreme electoral advantage. Although we find a few examples of Democratic gerrymanders with modest levels of bias, most of the extremely biased maps favor the Republican Party. In total, there are nearly two dozen maps that award Republicans 20 percent more of the seats than Democrats when the vote is close. These extreme partisan gerrymanders give Republicans a considerable structural advantage in state legislative elections. We estimate that, in the average state legislative assembly, Republican candidates can expect to win about 9 percent more seats than Democratic candidates would for a given share of the vote, between 45 percent and 55 percent of the vote.

We investigate the role of politics in determining whether a map is drawn with partisan bias. We take a systematic look at the processes that states use to regulate the drawing of the lines through case studies and empirical analyses of redistricting outcomes, paying particular attention to one key variable: whether the maps were drawn by politicians. We find that the most biased maps occur in competitive states where politicians from a single party were able to draw the maps without having to work with the minority party. Our examination of the effects of bipartisan districting yields inconclusive results. In some cases, when the redistricting process requires two parties to work together to pass a plan, redistricting authorities draw the maps with low levels of seat–vote responsiveness, presumably as a result of the drawing of “safe seats” to benefit incumbents from both parties. However, in other cases, we do not see this outcome. We do find that, on average, bipartisan districting leads to a preservation of the status quo. In the states where two parties worked together to draw the maps, the level of bias in the post-2011 plans deviated little from the pre-2011 plans.

Although scholars have investigated the effects of redistricting for many decades, surprisingly little is known about the causes of redistricting outcomes. We argue that studying state legislative redistricting in 2011 provides a unique opportunity to identify the theoretical determinants of partisan gerrymandering and assess the political, social, and democratic consequences of bias. We also argue that the story of 2011 redistricting is incomplete without a comprehensive investigation of the causes and consequences of redistricting of state legislatures. Ultimately, our findings reveal the interconnectedness of racial and political gerrymandering and suggest that partisan bias has long-term consequences for public policy and democratic health, yet they also show that it is possible to design redistricting institutions to prevent bias.