Computer models of the effects of gerrymandering on urban and rural voters

A new computer simulation of voting patterns by geography in the United States suggests gerrymandering may not be the cause of Republican majorities in the House:

To examine this hypothesis, we adapted a computer algorithm that we recently introduced in the Quarterly Journal of Political Science. It allows us to draw thousands of alternative, nonpartisan redistricting plans and assess the partisan advantage built into each plan. First we created a large number of districting plans (as many as 1,000) for each of 49 states. Then we predicted the probability that a Democrat or Republican would win each simulated district based on the results of the 2008 presidential election and tallied the expected Republican seats associated with each simulated plan.

The results were not encouraging for reform advocates. In the vast majority of states, our nonpartisan simulations produced Republican seat shares that were not much different from the actual numbers in the last election. This was true even in some states, like Indiana and Missouri, with heavy Republican influence over redistricting. Both of these states were hotly contested and leaned only slightly Republican over all, but of the 17 seats between them, only four were won by Democrats (in St. Louis, Kansas City, Gary and Indianapolis). While some of our simulations generated an additional Democratic seat around St. Louis or Indianapolis, most of them did not, and in any case, a vanishingly small number of simulations gave Democrats a congressional seat share commensurate with their overall support in these states.

The problem for Democrats is that they have overwhelming majorities not only in the dense, poor urban centers, but also in isolated, far-flung college towns, historical mining areas and 19th-century manufacturing towns that are surrounded by and ultimately overwhelmed by rural Republicans.

A motivated Democratic cartographer could produce districts that accurately reflected overall partisanship in states like these by carefully crafting the metropolitan districts and snaking districts along the historical canals and rail lines that once connected the nonmetropolitan Democratic enclaves. But such districts are unlikely to emerge by chance from a nonpartisan process. On the other hand, a Republican cartographer in these and other Midwestern states, along with some Southern states like Georgia and Tennessee, could do little to improve on the advantage bestowed by the existing human geography.

Perhaps this introduces a new strategy for political parties: the need to have more evenly distributed support rather than large clusters of support. But, as the bottom of the article notes, certain redistricting strategies like in Illinois or Maryland can provide Democrats some help in spreading out the effects of their urban voters.

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