Monday, February 13, 2017

Gerrymandering - no significant effect size

This goes against the received wisdom. Evaluating partisan gains from Congressional gerrymandering: Using computer simulations to estimate the effect of gerrymandering in the U.S. House by Jowei Chen and David Cottrell. From the Abstract:
What is the effect of gerrymandering on the partisan outcomes of United States Congressional elections? A major challenge to answering this question is in determining the outcomes that would have resulted in the absence of gerrymandering. Since we only observe Congressional elections where the districts have potentially been gerrymandered, we lack a non-gerrymandered counterfactual that would allow us to isolate its true effect. To overcome this challenge, we conduct computer simulations of the districting process to redraw the boundaries of Congressional districts without partisan intent. By estimating the outcomes of these non-gerrymandered districts, we are able to establish the non-gerrymandered counterfactual against which the actual outcomes can be compared. The analysis reveals that while Republican and Democratic gerrymandering affects the partisan outcomes of Congressional elections in some states, the net effect across the states is modest, creating no more than one new Republican seat in Congress. Therefore, the partisan composition of Congress can mostly be explained by non-partisan districting, suggesting that much of the electoral bias in Congressional elections is caused by factors other than partisan intent in the districting process.
Some interesting commentary at Marginal Revolution on this.

I like the methodology but have an instinctive doubt about the outcome. However, I cannot source the basis for that doubt. So I am skeptical of the finding but have no present reason to doubt it.

UPDATE: Two sources of doubt. One is that while there might only a marginal net effect at the system level, that doesn't reduce the flux at the local level. An economic analogy - If everyone lost their job in a year but were all able to find another job, we wouldn't believe that the economy was fine. The average net effect can mask localized churn which has second order consequences.

The second source of doubt is in regard to the likely increase in incumbency attendant to lower competition districts. Chen and Cottrell identify two types of gerrymandering. The first is the traditional partisan gerrymandering where a dominant party seeks to disfavor the smaller party by spreading their supporters among majority districts or, when that is untenable, concentrating all the smaller party voters into as few districts as possible.

The second form of gerrymandering is required via the Voting Rights Act which requires district boundaries to be gerrymandered along racial lines to ensure that African Americans are effectively, through gerrymandering, guaranteed a minimum of Congressional seats to African-American office holders.

The upshot of both these forms of gerrymandering is to reduce the inter-party competitiveness that would occur if district boundaries were randomly assigned. So the net impact of gerrymandering seats might be marginal but the number of seats that are competitive might be reduced.


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