Puzzle:
A single-winner election with honest voters and some given voting method will be called
"vulnerable to strategy" if there exist some subset of the voters, who, by changing
their votes, could cause the winner to change to somebody else whom that subset (unanimously)
prefers.
[This subset is not required to consist of identical voters; their honest
and/or strategic votes both are allowed to be multi-element sets.]
This puzzle investigates the probability that an election is vulnerable to strategy, in
the random election model (also called the "impartial culture")
with V→∞ voters.
By Warren D. Smith, Feb 2009; inspired by James Green-Armytage (whom I will abbreviate "J.G-A.")
a: is obvious... voting dishonestly plainly could hurt you but cannot help.
b,c,d,e,f,g: The "NES," i.e. "Naive Exaggeration Strategy," suffices to see these claims in view of standard properties of the normal distibution, and central limit theorem. Specifically, NES(X) means that the voters who prefer X>W change their vote by demoting W to last-ranked and promoting X to top-ranked, and otherwise leaving their ballots unaltered. Here W is the honest-votes winner. "NES" means NES(X) with whichever X causes it to work (i.e. succeed in changing the winner from W to X) if any X works. If no X≠W works we say NES "failed." It is not hard to argue NES(X) will work with probability→1 in all of these voting methods (for at least one X).
I don't want to give full details but will sketch a few cases. With plurality voting, all the N candidates have vote totals V/N equal to within relative error ±ε for any fixed ε>0 (with probability→1 when V→∞ with N held fixed) and then NES(X) strategy will cause X to win (for any X≠W) with probability→1.
With AntiPlurality and Coombs voting, NES(X) strategy will cause X to win (with probability→1) provided X is the candidate who would have won if all ballots ranking W bottom, magically vanished.
For median-based range voting, all N medians will be equal to within ±ε for any fixed ε>0 (with probability→1 when V→∞ with N held fixed). NES(X) strategy will raise X's and lower W's median by amounts of order 1/(N-1), which of course is much greater than arbitrarily small ε (with probability→1 when V→∞ with N held fixed).
An argument about strategy in Schulze-beatpaths voting may be found in my paper.
h,i: One reason the strategy vulnerability probability goes to 1 when N→∞ for IRV is the Quas-Smith theorem here.
The "NES," i.e. "Naive Exaggeration Strategy" suffices to prove lower bounds above 0 on strategy-vulnerability probability, but now does not yield 100%. Specifically we find for IRV that the election is (with probability→1) vulnerable to NES(X) strategy if X defeats W pairwise with honest votes, i.e. NES works if W is not a Condorcet winner. (But if W was a Condorcet winner, then with probability→1 W will defeat X in the final IRV round.) This happens (and does not happen) both with some nonzero constant probabilities when N=3. (And also when N=4,5,6,... but different constants for each value of N; see puzzle 102 for their values and the fact that when N→∞ the probability→1 that no Condorcet winner exists.)
For WBS-IRV and BTR-IRV: the election is (with probability→1) vulnerable to NES(X) strategy if X defeats W pairwise with honest votes, i.e. NES works if W had not been a Condorcet winner. NES(X) strategy may stop W from being a Condorcet winner even though it had been one with honest votes; but in that case X will not be a Condorcet Winner after NES(X) strategy (since it still loses pairwise to W) and hence X may or may not win the new election (it won't if W survives to the final round). Hence the limit strategy-vulnerability probabilities of BTR-IRV and WBS-IRV are lower bounded by their limit values for plain IRV, thus proving the "superiority" (or at least equality) of WBS-IRV and/or BTR-IRV over plain IRV.
The fact that limit-values exist is a consequence of the abstract theory of Schläfli functions which indeed in principle allows expressing them (at least for small N) in closed form. The fact that they are bounded above 0 has already been discussed. Indeed, the upper bound on the invulnerability probability for IRV is precisely the product of P(N), i.e. the probability that a Condorcet winner exists among N candidates (computed in this puzzle), times the conditional probability the IRV and Condorcet winners agree conditioned on the CW existing – which has been evaluated numerically in table 14 here. The result is:
James Green-Armytage claims, using a new mathematical-programming formulation (building on 1980s work of J.R.Chamberlin) and a computer, to have computed the probability, in 999-voter random elections, that any voter subset can use any strategy to change the IRV winner in a way that they (unanimously) consider to be an improvement. (Note, this is not just the NES subclass of strategies, it's all strategies.) He computed IRV's vulnerability-to-strategy probabilities as 16%, 33%, and 49% for the 3, 4 and 5-candidate cases, respectively. [And these indeed are greater than our NES-based lower bounds 12%, 23%, 33%, as they should be, and indeed appear to be rising toward 100%, as the theorem says they must.]
It would be interesting to know what J.G-A.'s computer says about WBS-IRV and BTR-IRV???
Remark: It seems if you want an elimination method to have good chances of immunity to NES-type strategy, then eliminations must be based on top-rank votes only; if they also involve any other rank, then you get vulnerability to NES with probability→100%. So IRV and methods like it such as WBS-IRV seem singled out by this criterion.
To upperbound the vulnerable-to-strategy limit-probabilities below 1, we need to find a positive-probability subclass of elections which are immune to strategy. I can do that for IRV 3-candidate elections as follows. We showed vulnerability to NES strategy 12% of the time. The only other kind of strategy one could imagine in a 3-candidate election would be in non-monotone IRV elections – but these are only 14.7% of random-model elections. Hence we get vulnerability to any strategy at most 28% of the time (28=14.7+12+safety margin) for IRV 3-candidate elections. [Note this also is obeyed by J.G-A's numerical finding of 16%.] One could also use an analogous approach to prove lower bounds below 1 for various other election methods such as WBS-IRV and BTR-IRV (and also perhaps for N=4 or N=5 candidates too).
J.G-A. also notes that "most other methods" (besides IRV) exhibit strategy-vulnerability chances near 100%. This computer result also confirms our theoretical results.
j: One may also show positive lower bounds on the vulnerable-to-strategy limit-probabilities from the quantitative Gibbard-Satterthwaite theorem by Ehud Friedgut, Gil Kalai, and Noam Nisan 2008 (pdf of their paper); this works for a very wide class of election methods.
k: Criticism of the "vulnerability to strategy" probability as a yardstick for comparing voting systems: What matters to civilization is not the probability that some voter-subset could strategize; what matters is the expected utility-decrease caused by the net effect of all strategic behaviors undertaken by all classes of voters. Some of these strategic moves will reinforce, whereas others will "cancel out." Certainly it is utterly unrealistic to pretend the pessimal voter subset colludes based on perfect information about the noncolluders – while all other voters (not in that subset) forswear all strategy! And it is also utterly ludicrous to pretend all strategy-caused winner-changes are "equally bad!"
(In short, the right yardstick for measuring vulnerability to strategy, as well as lots of other things too, instead is Bayesian Regret.)
Also the "random election model" itself is subject to criticism since it is not very realistic.
If strategic voters have to act on imperfect information, the situation also changes. For one thing, they might try a strategy which fails to work out for them, but still causes damage – and such damage ought to be counted, not ignored.
Example I (Range superior to IRV under a different definition of "vulnerability to strategy"): I have argued elsewhere that if all the voters know is that "in IRV elections third party candidates are historically very unlikely to win" (and as of 2009 this has historically been true in every IRV country, e.g. as of 2009 Australia has zero third-party members in its IRV-elected house) then they would seem justified in "favorite betrayal" for third-party candidates 100% of the time! The argument is that that candidate is very unlikely (well below 1%) to be able to win (regardless of how they strategize), in which case such betrayals cost nothing. However, random-model IRV elections in about 20% of 3-candidate cases have the property that the votes for the 3rd-party candidate top, cause the "greater evil" to win (if they'd betrayed him, the lesser evil would have won). In that case the betrayal gains the voter something.
So in this other sense, IRV elections have 100% probability of "vulnerability to strategy" whereas range-voting elections are entirely immune to favorite-betrayal!
Example II (Condorcet superior to IRV under a different probability model – 1D politics): In a different model, namely "one dimensional politics (singlepeaked utility functions for candidates lying along a line) 3-candidate elections" IRV is fairly likely to fail to elect a centrist Condorcet winner – indeed here is a model in which this failure-probability is 1/3 – and in every such situation is "vulnerable to strategy" because the losing extremist-faction can betray their favorite to elect the centrist.
But meanwhile for Condorcet methods: a Condorcet winner always exists in 1D politics in this model, neglecting ties (Duncan Black's singlepeakedness theorem) and Condorcet methods are always invulnerable to strategy in 3-candidate 1D-politics elections.
Proof sketch: If the CW is the centrist, then if either extremist faction strategically buries the centrist, it'd make the wrong extremist win or cause no change. If the CW is an extremist, then he automatically is a majority-top winner and so if the centrist faction artificially buries him (the other extremist faction already is ranking him bottom) it won't matter. Q.E.D.
So Condorcet methods are actually optimal within this puzzle's "vulnerability to strategy" definition – zero probability of "vulnerability to strategy" – in 3-candidate 1D-politics elections, while IRV is nonoptimal! That's exactly the opposite of JGA's conclusion in a different probabilistic model.
Example III (Range superior to IRV under a different model – 1D politics – and a different definition): As we saw here range voting with strategic voters will tend to yield honest-voter Condorcet winners (and do so more often than IRV). Hence in the 1D politics model in example II, we could argue range voting too is "superior" to IRV.