Abstract
This paper presents a new measure of gerrymandering, called the Reverse Gerrymandering Index (RGIx), and uses it to analyze the evolution of packed districts in the US House of Representatives from 1872 to 2022. Unlike the efficiency gap and partisan symmetry measures, which are influenced by vote variation across the entire state, the mainstay of RGIx is that it estimates the gerrymandering level of each district by comparing its partisan vote distribution with the partisan vote distribution of the districts adjacent to it. From there, RGIx can be used to identify highly gerrymandered districts, measure regional or historical variation in gerrymandering, or as a variable in statistical models, to name just a few applications. It can also calculate gerrymandering scores for an unlimited number of parties per district, making it applicable to comparative electoral systems research. In this paper, I use RGIx to show that there were few packed congressional districts for nearly a century after the American Civil War, that gerrymandering began rising rapidly in federal elections in the 1960s, and that it began to shift dramatically in a pro-Republican direction starting in the 1980s.