July 7th, 2010

ni

On gerrymandering

Sparked by yesterday's discussion, this is cut and pasted from my website, and records a discussion between me, Jim Riley and others on Usenet (remember that?) back in August 1999, demonstrating that requiring a rigid ratio between electors and representatives is not a safeguard against gerrymandering; if anything, the reverse.

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Of course this is all a bit unrealistic in two respects. First, nowhere in the world insists on such a degree of accuracy in the ratio between representatives and population/electorate (and we'll just ignore that last distinction for now). Populations move all the time in the real world, and tolerances of 5% within a given region are usual; 10% is not unreasonable. The more members per constituency, the easier it is to make a difference.

But second, which largely cancels out the first point, with margins as tight as these, the gerrymanders are very vulnerable to differential turnout and even more so to people of one tribe voting for candidates from another, or failing to transfer their votes down the line.

It is intrinsically more difficult to gerrymander proportional systems to get the result you want. Usually all you can hope to affect is where the 'last seat' in each constituency goes.

The problem with any single-seat system is that every seat is the last seat in that constituency!