In reviewing the extensive entries about the recent UK electoral system referendum, which had been posted at the LSE blog on British politics over the past year, I was struck by Professor Patrick Dunleavy’s claim that the Supplementary Vote would have been a fairer system to propose. The Supplementary Vote (SV) is another form of “instant runoff,” and thus bears considerable affinity to the Alternative Vote (AV)–the option that actually was put up against the status quo in last week’s referendum (and was defeated resoundingly).
Under SV, voters are allowed to give just two preferences. If no candidate has a majority of first-preference votes, then all candidates in the contest other than those with the top two totals of first preferences are eliminated. Second choices are then redistributed among the top two, and the candidate with the most votes at this stage is elected.
This procedure differs from AV in at least two fundamental respects. First of all, under AV the voter may (or if we are talking about the Australian House of Representatives, must) rank all the candidates, not just two. Second, AV is a sequential elimination method. Rather than eliminating all but the initial top two, it eliminates the weakest candidate at each stage of the count, up until a winner has been determined. AV thus emulates, in one “instant” act of ballot-marking, the sequential processes used in many political parties and legislative institutions to choose a leader, rather than the two-round majority rules used to elect many national presidents, as does SV.1
SV is used to elect the London mayor and those of some other English cities; it is used nowhere to elect legislators that I know of.
In a post from last summer, Dunleavy stated:
The London form of AV also has the great advantage that it creates a run-off between the top two candidates in a local constituency – only one of them can win.
I must admit to having a hard time understanding why this is a virtue for a ranked-ballot method. If the race has two clear front-runners and one or more also-rans, fine. The two systems would produce the same result, and such a result would be “fair” by almost any standard, I should think. However, if there is uncertainty about the top two as the election day looms, SV poses a real problem for voters. Not only must they know their two top preferences (easy enough), but they must also think strategically about them (potentially a challenge). If one’s first preference is likely to place third or lower, then one must be careful that one’s second preference will clear the initial top two. Otherwise, the second preference will not be counted, as it will have been wasted on an eliminated candidate. By contrast, under AV, if your second preferred candidate has been eliminated, the counters look to your third (and so on).
But isn’t SV a truer (instant) runoff, inasmuch as it is a top-two system? Here is where the instant-ness ceases to be an advantage. In an actual two-round system of majority runoff, the top two choices of the electorate as a whole are revealed before the voters must give their second preference (i.e. before voters whose initial choice was eliminated in the first round decide which of the two remaining candidates to support in the runoff). The risk of a relatively more extreme candidate winning would be greater under SV than under either AV or two-round majority: if more moderate voters lost their second preferences by tending to have cast them on the third-place candidate, relatively more extreme voters may elect, by less than 50%, their preferred candidate. I have no idea how common this would be in practice, but it seems it would be a not implausible effect of SV. In an actual second round, under this scenario, the voters can “update” and vote for the less extreme of the remaining candidates in the runoff. 2
Given the substantial number of UK constituencies in which there is a close race between second and third, SV would seem to be clearly less fair than AV to voters confronted with strategic choices about estimating which two candidates are the top two of the electorate at large. (By the way, it is also the case that in presidential elections by two-round majority, the gap between the second and third candidates tends systematically to be less than that between the top two, suggesting that SV would indeed be a poor choice for these elections.)
I fail to see why a case could be made that SV is fairer, especially in any setting in which the “Duvergerian” tendencies of an existing FPTP (plurality) system have already broken down–which are pretty much the only situations in which it would be considered in the first place.
- Intermediate variants are also possible. For instance in some US jurisdictions that have adopted forms of instant runoff, voter preferences are restricted to the top three. The counting process in these jurisdictions typically otherwise follows the AV approach, rather than that of SV (as far as I know; there may be exceptions).
There is also the so-called Contingent Vote, which allows voters to express more than two preferences, but shares with SV the immediate elimination of all but the top two candidates if none has a majority. Thus the Supplementary Vote and the Contingent Vote arguably have more in common with one another than either has with the Alternative Vote, although the Contingent Vote is less likely to waste votes of those who gave their second preference to an already eliminated candidate. (A form of the Contingent Vote seems to be what Sri Lanka uses to elect its president.) [↩]
- I recall this point being addressed somewhere in the political science literature; apologies for not having a citation. [↩]