“The Supreme Court is deciding a gerrymandering case. Here’s the social science that the justices need to know.”

Chris Warshaw for Monkey Cage:

There are several ways to measure partisan advantage in a districting plan, including the efficiency gappartisan symmetrydeclination and the mean-median difference. These metrics all capture distortions in the vote-seat relationship. In a recent work, Nicholas Stephanopoulos and I averaged these measures to estimate the partisan bias in each state’s districting process — that is, whether these distortions advantaged one party. The graph below draws on data from all congressional elections from 1972 to 2018 in states with more than six congressional seats.

The graph shows just why partisan gerrymandering has become so controversial: the states that have produced litigation really do have historically high levels of partisan bias in their congressional maps. Michigan, Ohio, Pennsylvania and North Carolina have some of the largest pro-Republican biases in the past 40-plus years, and Maryland has a large pro-Democratic bias. For instance, Ohio’s congressional map had a higher level of pro-Republican bias in 2012 than 99.5 percent of previous congressional maps.


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