I mentioned before that I wanted to try expanding the range of things I blog about here. One example is that I have a long-standing interest in game theory, which I think began when I was an undergrad at U of Waterloo. I don’t (currently) do research in game theory, and have nothing particularly novel to say about it (though naturally a default question for me would be: can you categorify it?), but it is regularly used to analyze situations in evolutionary theory, and social sciences like economics, and politics. So it seems to be a fairly fundamental discipline, and worth looking at, I think.
For a little over a week now, Canada has been in a campaign leading up to a federal election in October. Together with the constant coverage of the US election campaign, this means the news is full of politicking, projections of results, etc. Also as usual, there are less-well-covered people agitating for electoral reform. So it seemed as good a time as any to blog something about social choice theory, which is an application of game theory to the special problem of making choices in groups which reflect the preferences of its members.
This is the defining problem for democracy, which is (in recent times) the world’s most popular political system, so it has been pretty extensively studied. One aspect of democracy is voting. One aspect of social choice theory is the study of voting systems – algorithms which collect information about the populations preferences among some alternatives, and produce a “social choice function”. For example, the US Presidential election process (in its ideal form, and excluding Primaries) collects the name of one candidate from each voter, and returns the name of a single winner. The Australian process collects more information from each voter: an ordered list of candidates. It uses a variant of the voting scheme called STV, which can return more than one winner.
Okay, so there are many different voting methods. Why should there be so many? Why not just use the best? For one thing, there are different criteria what is meant by “best”, and every voting system has some sort of limitation. Depending on your precise criteria, you may prefer one system or another. A more precise statement of this comes in the form of Arrow’s theorem.
Suppose we have a set of “choices” (a general term for alternatives, not necessarily candidates for office), and for each voter in the population of voters , there is , a total order on (a “preference order”). Call this information a “profile”. Then we’d like to collect some information about the profile (perhaps the whole thing, perhaps not), and produce a “social preference order” . This is a function (where is the set of orders on ). The functon should satisfy some conditions, of course, and many possible conditions have been defined. Arrow’s theorem is usually stated with five conditions, but an equivalent form uses these:
- Unrestricted Domain: is surjective. (I.e. any preference order could potentially occur as output of the algorithm – e.g. in a single-winner election, the system should allow any candidate to win, given enough votes)
- Independence of Irrelevant Alternatives: for any , the restriction agrees with (i.e. when applied to the restrictions of the orders to , it produces the restriction of to . In particular, removing a non-winning candidate from the ballot should not affect the outcome.)
- Monotonicity: if prefers alternative to , then changing any so that it prefers to (assuming it did not) should not cause to prefer to (i.e. no voter’s ballot has a harmful effect on their preferred candidate).
Arrow’s theorem says that no algorithm satisfies all three conditions (and of course some algorithms don’t satisfy any). (A different formulation and its proof can be found here.)
Most popular voting systems satisfy (1) and (3) since these are fairly obvious criteria of fairness: every candidate has a chance, and nobody’s votes have negative weight (though note that a popular reform suggestion in the US, Instant Runoff Voting, fails monotonicity!). Condition (2) is the one that most people seem to find non-obvious. Failures of condition (2) can take various forms. One is “path dependence”, which may occur if the decision is made through a number of votes, and the winner can (for some profiles) depend on the order in which the votes are taken (for instance, the runoff voting used in French presidential elections is path dependent). When a voting system has this property, the outcome can sometimes be manipulated by the “agenda-setter” (if there is one).
Another way (2) can fail is by creating the possibility of strategic voting: creating a situation in which voters have a rational incentive to give false information about their preferences. For instance, voters may want to avoid “splitting” the vote. In the US election in 2000, the presence of the “irrelevant” (i.e. non-winning) alternative Ralph Nader (allegedly) split the vote which otherwise would have gone to Gore, allowing Bush to win certain states. Somewhat similarly, in the current Canadian election, there is a single “right wing” party (Conservative), two or three “left wing” parties, depending on the riding (Liberal, NDP, Bloc Quebecois), and one which is neither (Green). In some competitive districts, for example, voters who prefer NDP to Liberal, but Liberal to Conservative, have a rational reason to vote for their second choice in order to avoid their third choice – assuming they know the “real” race is between Conservative and Liberal in their riding. (In my riding, North London Centre, this is not an issue since the outcome – Liberal – is not in doubt at all.)
These, like all voting system flaws, only become apparent when there are at least three options: with only two options, condition (2) doesn’t apply, since removing one option leaves nothing to vote on. (This is one reason why many voting systems are said to “favour the two-party system”, where its flaws are not apparent: when the vote is split, voters have an incentive to encourage parties to merge. This is why Canada now has only one “right-wing” party).
These two flaws also allow manipulation of the vote only when the manipulator knows enough about the profile of preferences. Apart from allowing parties to find key competitive ridings (or states, etc.), this is probably one of the most important uses of polling data. (Strategic voting is hard in Canada, since a good statistical sample of, say, 1000 in each of 308 ridings would require polling about 1% of the total population of the country, so one usually has only very imperfect information about the profile. Surveying 1000 people in each of 50 US states is relatively much easier. Even at that, projections are hard: try reading any of the methodology details on, say, fivethirtyeight.com for illustration.)
Now, speaking of strategic voting, the Gibbard-Satterthwaite theorem, closely related to Arrow’s theorem, applies to social choice systems which aim to select a single winner based on a number of votes. (Arrow’s theorem originally doesn’t specifically apply to voting: it applies to any multi-criterion decision-making process). The G-S theorem says that any (deterministic) voting system satisfies one of three conditions:
- It is dictatorial (i.e. there is a voter $v$ such that the winner depends only on $P_v$
- It is restricted (i.e. there is some candidate who cannot win no matter the profile)
- It allows strategic voting for some profiles
So it would seem that the possibility of strategic voting can’t reasonably be done away with. This suggests the point of view that voting strategically is no more a problem than, say, playing chess strategically. The fact that an analysis of voting in terms of the theory of games of strategy suggests this point of view is probably not a coincidence…
As I remarked, here in London North Centre, the outcome of the vote is in no doubt, so, strategically speaking, any vote is as good as any other, or none. This curious statement is, paradoxically, only true if not believed by most voters – voting strategically in a context where other voters are doing the same, or worse yet, answering pollsters strategically, is a much more complicated game with incomplete (and unreliable) information. This sort of thing is probably why electoral reform is a persistent issue in Canada.