IRV and Core Support

IRV ignored such a winner in a 2009 IRV mayoral race in Burlington, Vermont. In that election, Democrat Andy Montroll was favored over Republican Kurt Wright 56% to 44% (930-vote margin) and over Progressive Bob Kiss 54% to 46% (590-vote margin), majorities in both cases. In other words, in voting terminology, Montroll was a “beats-all winner,” also called a “Condorcet winner” – and a fairly convincing one.

However, in the IRV election, Montroll came in third. Kiss beat Wright in the final IRV round with 51.5% (252-vote official margin).

But some excuse the fact that IRV can fail to elect a Condorcet winner in such cases, claiming that the Condorcet winner didn’t have enough first-place votes. Advocates call these first-place votes “core support”. Here’s one perspective of that claim:

“Condorcet-type voting violates the principle of requiring a minimum level of core support by permitting a candidate to win who would not win a single vote in a plurality election.”

Contra these claims, below is a simplified IRV election scenario. One of the losing candidates is preferred by a majority of voters to the winning candidate, and receives more “core support” than the winner.

% of voters         their ranking
35%                   W > Y > Z > X
17%                   X > Y > Z > W
32%                   Y > Z > X > W
16%                   Z > X > Y > W

Instant Runoff Voting selects candidate X as the winner, beating W in the final round, 65% to 35%.

But is that really a majority? A huge 67% majority of voters would rather have candidate Y than X. And Y received nearly twice as many first-place votes as X, 32% vs. 17%.

And an even larger 83% super-majority of voters would rather have candidate Z than X (and Z got just a little fewer first-place votes than X).

So the claim that IRV “elects majority winners” is seriously misleading.

Also…

The first row of voters have an incentive to betray W by pretending Y is their actual favorite – then they get their second choice instead of their last. W is a spoiler. If he would drop out of the race, then Y would win instead, even with no change in voter preferences.

The third row of voters have an incentive to betray candidate Y by pretending candidate Z is their favorite – then they get their second choice instead of their third.

The first row of voters made a big mistake by voting honestly. Suppose 20% of the voters, all from that bloc, had simply refused to vote. That would actually have been better for them than voting honestly, because it would have caused Y to win (whom they prefer over X). Their honest “X is worst” votes actually caused X to win.

Also, Y is the Condorcet “beats-all” winner, but doesn’t make it to the final round: 65% majority says Y>W; 67% majority says Y>X; 84% majority says Y>Z.

And W is the Condorcet “lose-to-all” loser, but makes it to the final round (65% majorities say others>W).