Friday, November 20, 2009

A Majority of a Majority is a Minority

Consider an election in a swing district; one that is very nearly evenly split between voters who generally prefer Democrats over Republican and voters who generally prefer Republicans over Democrats. Consider also that, in nearly all districts, voters can vote in a primary election, but in only one of the two primaries. Unless the primaries are complete blowouts, the winner of the election can not possibly be the favorite of a majority of voters. Rather, they will be the favorite of a majority of the majority, which could be as little as 25%+1 of the voters—or less if there are more than two candidates in the primary!

Score voting is occasionally attacked because it fails the "majority criterion", which states that, if a candidate is the first-choice of a majority of the voters, that candidate should win the election. I'll provide a simple example: three candidates, A, B, and C. Three voters, two of which vote A=9, B=5, C=0, and one who votes A=0, B=9, C=9. A majority of the voters (two out of three) prefer A over B, but B wins the election, with an average score of 6.33 versus A's score of 6.00. Score voting fails the majority criteria. (This might not be a bad thing, but that's a post for another day...)

But consider this in context of the first paragraph's example: plurality elections are known to fail spectacularly when there are more than two strong candidates, which is why the system of party primaries developed. But this has the side effect of squeezing the middle out of the pool of candidates. While it's true that many primary voters will take into consideration a candidate's chances of winning the general election, the voters are still by and large trying to choose a candidate who is preferable only to the majority of the party members; then, the hope is that the party members will be a majority of the electorate. However, we must remember that this candidate was the first-preference of only a majority of a majority. And since that's the case, is it even meaningful to say that plurality elections (preceded by primaries) choose a "majority winner"?

If you were to put all the candidates from both parties' primaries up at once, how often would more than half of all the voters indicate a single one of those candidates as their favorite? Not very often! Surely, if there were such a candidate, they would win any subsequent plurality election, but when we say that plurality elects a "majority winner", we are usually not talking about such a candidate, even if the system we use obscures this fact from us.

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