In the fields of mechanism design and social choice theory, Gibbard's theorem is a result proven by philosopher Allan Gibbard in 1973. It states that for any deterministic process of collective decision, at least one of the following three properties must hold:
- The process is dictatorial, i.e. there exists a distinguished agent who can impose the outcome;
- The process limits the possible outcomes to two options only;
- The process encourages agents to think strategically: once an agent has identified their preferences, they have no action at their disposal that would best defend their opinions in any situation.
A corollary of this theorem is Gibbard–Satterthwaite theorem about voting rules. The main difference between the two is that Gibbard–Satterthwaite theorem is limited to ranked (ordinal) voting rules: a voter's action consists in giving a preference ranking over the available options. Gibbard's theorem is more general and considers processes of collective decision that may not be ordinal: for example, voting systems where voters assign grades to candidates.
Gibbard's theorem is itself generalized by Gibbard's 1978 theorem and Hylland's theorem, which extend these results to non-deterministic processes, i.e. where the outcome may not only depend on the agents' actions but may also involve a part of chance.
Consider some voters , and who wish to select an option among three alternatives: , and . Assume they use approval voting: each voter assigns to each candidate the grade 1 (approval) or 0 (disapproval). For example, is an authorized ballot: it means that the voter approves of candidates and but disapproves of candidate . Once the ballots are collected, the candidate with highest total grade is declared the winner. Ties between candidates are broken by alphabetical order: for example, if there is a tie between candidates and , then wins.
Assume that voter prefers alternative , then and then . Which ballot will best defend her opinions? For example, consider the two following situations.
- If the two other voters respectively cast ballots and , then voter has only one ballot that leads to the election of her favorite alternative : .
- However, if we assume instead that the two other voters respectively cast ballots and , then voter should not vote because it makes win; she should rather vote , which makes win.
To sum up, voter faces a strategic voting dilemma: depending on the ballots that the other voters will cast, or can be a ballot that best defends her opinions. We then say that approval voting is not straightforward : once the voter has identified her own preferences, she does not have a ballot at her disposal that best defends her opinions in all situations.
Unfortunately, Gibbard's theorem states that a deterministic process of collective decision cannot be straightforward, except possibly in two cases: if there is a distinguished agent who has a dictatorial power, or if the process limits the outcome to two possible options only.
Let be the set of alternatives, which can also be called candidates in a context of voting. Let be the set of agents, which can also be called players or voters, depending on the context of application. For each agent , let be a set that represents the available strategies for agent ; assume that is finite. Let be a function that, to each -tuple of strategies , maps an alternative. The function is called a game form. In other words, a game form is essentially defined like a game, but with no utilities associated to the possible outcomes: it describes the procedure only, without specifying a priori the gain that each agent would get from each outcome.
We say that is straightforward if and only if for any agent and for any strict weak order over the alternatives, there exists a strategy that is dominant for agent when she has preferences : there is no profile of strategies for the other agents such that another strategy , different from , which would lead to a strictly better outcome (in the sense of ). This property is very desirable for a democratic decision process: it means that once the agent has identified her own preferences , she can choose a strategy that best defends her preferences, with no need to know or to guess the strategies chosen by the other agents.
We denote by the range of , i.e. the set of the possible outcomes of the game form. For example, we say that has at least 3 possible outcomes if and only if the cardinality of is 3 or more. Since the strategy sets are finite, is finite also; thus, even if the set of alternatives is not assumed to be finite, the subset of possible outcomes is necessarily so.
We say that is dictatorial if and only if there exists an agent who is a dictator, in the sense that for any possible outcome , agent has a strategy at her disposal that ensures that the result is , whatever the strategies chosen by the other agents.
We assume that each voter communicates a strict weak order over the candidates. The serial dictatorship is defined as follows. If voter 1 has a unique most-liked candidate, then this candidate is elected. Otherwise, possible outcomes are restricted to his ex-aequo most-liked candidates and the other candidates are eliminated. Then voter 2's ballot is examined: if he has a unique best-liked candidate among the non-eliminated ones, then this candidate is elected. Otherwise, the list of possible outcomes is reduced again, etc. If there is still several non-eliminated candidates after all ballots have been examined, then an arbitrary tie-breaking rule is used.
This game form is straightforward: whatever the preferences of a voter, he has a dominant strategy that consists in declaring his sincere preference order. It is also dictatorial, and its dictator is voter 1: if his wishes to see candidate elected, then he just has to communicate a preference order where is the unique most-liked candidate.
Simple majority vote
If there are only 2 possible outcomes, a game form may be straightforward and not dictatorial. For example, it is the case of the simple majority vote: each voter casts a ballot for her most-liked alternative (among the two possible outcomes), and the alternative with most votes is declared the winner. This game form is straightforward because it is always optimal to vote for one's most-liked alternative (unless one is indifferent between them). However, it is clearly not dictatorial. Many other game forms are straightforward and not dictatorial: for example, assume that the alternative wins if it gets two thirds of the votes, and wins otherwise.
A game form showing that the converse does not hold
Consider the following game form. Voter 1 can vote for a candidate of her choice, or she can abstain. In the first case, the specified candidate is automatically elected. Otherwise, the other voters use a classic voting rule, for example the Borda count. This game form is clearly dictatorial, because voter 1 can impose the result. However, it is not straightforward: the other voters face the same issue of strategic voting as in the usual Borda count. Thus, Gibbard's theorem is an implication and not an equivalence.