Gerrymandering and the net number of US House seats won due to vote-distribution asymmetries

TL;DR

This paper uses a new declination function to estimate how vote distribution asymmetries, due to gerrymandering and geographic factors, have historically biased US House seat outcomes, revealing partisan advantages over time.

Contribution

It introduces the declination function as a novel method to quantify vote asymmetries and biases in US House elections, validated through simulation techniques.

Findings

Significant Democratic bias before mid-1990s

Republican bias from 1990s onward

Net seat advantages of 28, 20, and 25 for Republicans in 2012, 2014, 2016

Abstract

Using the recently introduced declination function, we estimate the net number of seats won in the US House of Representatives due to asymmetries in vote distributions. Such asymmetries can arise from combinations of partisan gerrymandering and inherent geographic advantage. Our estimates show significant biases in favor of the Democrats prior to the mid 1990s and significant biases in favor of Republicans since then. We find net differences of 28, 20 and 25 seats in favor of the Republicans in the years 2012, 2014 and 2016, respectively. The validity of our results is supported by the technique of simulated packing and cracking. We also use this technique to show that the presidential-vote logistic regression model is insensitive to the packing and cracking by which partisan gerrymanders are achieved.