June 1, 2006

Does gerrymandering really impact electoral success?

Two Harvard economists contend that the answer is no:

How can we determine whether gerrymandering is the culprit, especially since any number of reasons could account for the increase in the incumbent reelection rate? The key is that redistricting usually happens only once each decade (at least until the recent controversy in Texas). Other factors, such as money or electoral polarization, tend to change more smoothly over time. One can tease these factors apart with a "regression discontinuity" approach, separating the changes over time into smooth year-on-year increases and jumps at the time of gerrymandering.For instance, consider the fate of incumbents over the past decade or so. Though fewer than 2 percent of incumbents were defeated in 2002--the election immediately following redistricting--the rate was just as low in 1998, 2000, and 2004. The redistricting before the 2002 election does not seem to have affected incumbents at all. On the other hand, incumbents suffered more than twice that number of defeats in each election between 1992 and 1996, exactly the years when the gerrymandering before 1992 should have been most effective. What's more, these years were far worse for sitting congressmen than those just before the 1991 gerrymander, in which incumbents fared almost as well as they do today.Of course, a thorough study must examine not just these few elections but also the longer trends over the past 30, or even 100, years. In a recent paper we perform such an analysis. We find that changes in redistricting have actually slightly reduced incumbent reelection rates over time. Instead, it looks like the other factors we mentioned above--money, polarization, media exposure, and perhaps others--account for the fall in electoral competitiveness in Congress.
The full paper is availble here. Really cool stuff.