In this chapter, we develop a general model of the redistricting process in the United States. We use this model to examine two features of how congressional votes translate into seats: partisan bias (how much larger or smaller a party's seat share is than its vote share would warrant) and responsiveness (how much party seat shares respond to changes in vote shares). Bias and responsiveness are standard concepts in the analysis of redistricting and will be defined more fully later. From our model, we derive specific hypotheses about how the bias and responsiveness of a redistricting plan will differ as a function of two conditions obtaining when that plan is enacted: (1) the legally defined reversionary outcome of the redistricting process and (2) which party controls the legislative branches (house, senate, and governorship) of the state. We test our hypotheses in the next chapter.

a model of congressional redistricting

The literature on redistricting categorizes gerrymanders according to the varying goals that those who redraw district lines pursue. For present purposes, the most important categories are proincumbent gerrymanders (when a bipartisan alliance draws the lines to preserve the current incumbents' chances of victory) and partisan gerrymanders (when a single party draws the lines to maximize its seat share). In this section, we develop a model in which these two types of gerrymander – along with a third “mixed” type – emerge endogenously as a function of partisan control of the redistricting process and the nature of the reversionary outcome.

The basic elements of our model are straightforward. There are two parties, the Republicans and the Democrats, in a given state.