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COMPUTING SCIENCE

Machine Politics

Brian Hayes

Remaking the House

The U.S. House of Representatives reconstitutes itself every 10 years in a two-part process. The first part is reapportionment, in which each state is allotted its share of representatives. This is a purely arithmetical operation, and at first it appears quite simple. Each state's share of the seats in the House is proportional to its share of the total population, as enumerated in the decennial census. But representatives come only in integer units, and so some rounding of numbers is needed. That's where the trouble lies. There are many schemes for rounding, some of which favor the larger states and some the smaller.

From 1790 until 1940, reapportionment was a reliable source of Congressional acrimony every 10 years. Since 1941, remarkably, it has become a nonissue. What made the difference was writing an algorithm into the law and thereby making the reapportionment process automatic. Once the census returns are in, the allocation of seats to states is immediately determined just by cranking the numbers through the algorithm. There is no room for human judgment or discretion. As it happens, the algorithm chosen is probably not the best one for the purpose, but no major faction has been sufficiently disgruntled to mount a serious challenge. This absence of discord is perhaps the one bit of empirical evidence suggesting that algorithmic methods might really have something to offer to political science.

The second part of the decennial legislative makeover is redistricting. Once a state learns that it will have k seats in Congress, it must divide its territory into k districts, each of which will elect one member. This is not an arithmetic procedure but a geometric one, and it is severely underconstrained. For any k > 1, there are many, many ways of drawing lines on the map to create k districts. This superabundance of solutions is an invitation to political (or meta-political) mischief.

The kind of mischief that comes to mind first is the partisan gerrymander—a redistricting plan that favors one political party over another. The term dates back to 1812, when Elbridge Gerry, as governor of Massachusetts, presided over the creation of a sinuous legislative district that opponents compared to a salamander. There is also the "sweetheart" gerrymander, where incumbents of opposing parties collude on a redistricting plan to ensure their own re-election. Political gerrymandering is roundly condemned by civic-interest groups, who argue that it gives an unfair advantage to those who happen to be in control in a year divisible by 10. They get to rig the electoral machinery to retain their advantage throughout the following decade.

The legal status of political gerrymandering is uncertain. No federal statute explicitly forbids it, and so far the Supreme Court has never overruled a districting plan because of political bias. Nevertheless, gerrymandering is widely considered unsporting, and blatant instances are likely to offend the electorate.

Figure 1. North Carolina's 12 Congressional districts . . .Click to Enlarge ImageAnother form of gerrymandering, which aims to dilute the political strength of racial and ethnic minorities, was outlawed by the Voting Rights Act of 1965. Measures taken to comply with this act were the issue in the recent North Carolina and Texas litigation. Both states, under instruction from the Department of Justice, created "minority-majority districts" where African-American or Hispanic communities would be able to elect representatives of their choice. The Supreme Court ruled four of these districts unconstitutional on the grounds that race or ethnicity had been the predominant consideration in drawing them. (Note: I live and vote in the soon-to-be-former 12th district of North Carolina, and the recent commotion over its status is what inspired me to write on this theme. North Carolina districts serve as examples in much of the discussion that follows.)





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