Strategic nomination is the manipulation of an election by its candidate set. It is different from tactical voting in which the manipulation comes from the voters. It is different also from campaign strategy, the methods candidates employ in political campaigns to win an election after the nomination.
Independence of irrelevant alternatives
If the winner of an election were not running in the first place, then obviously someone else would have won instead. Similarly, if a candidate gets "added" to an election, it becomes possible for the new candidate to win. If these are the only cases in which a change in the candidate set leads to a different election outcome, then the voting system is independent of irrelevant alternatives and therefore immune to strategic nomination.
Independence of irrelevant alternatives, however, is a very hard property for a voting system to satisfy. This is illustrated by the following example of Condorcet's voting paradox:
- 40 voters preferring candidate A to B to C
- 35 voters preferring candidate B to C to A
- 25 voters preferring candidate C to A to B
With the above preferences and whatever candidate an election method chooses as a winner, another candidate can always secure a majority of votes against that winner by removing the third candidate. Since the absence of any candidate would leave the impression that the preference of the group of voters as a whole is a clear majority when by definition it is not when we consider the third candidate, one can argue that none of these candidates are actually "irrelevant."
The candidates in the example above form a cycle known as the Smith set - their combined presence provides conflicting information (both to the election system as well as to observers) about who the greatest candidate is. Strategic nomination, then, involves hiding this information from the voting system by excluding one of the candidates. Because of this strange relationship between the candidates and the voters, strategic nomination through this manner is doubtful as it becomes very much a question of whether the presence or absence in an election of a potential "cycle-maker" (provided one exists and can be found) can be decided by those who seek to gain from it.
Independence of clones
In order to simplify the issue, academic attention sometimes focuses on a specific kind of strategic nomination: the kind that involves clones. Clones in this context are candidates such that every voter ranks them the same relative to every other candidate, i.e. two clones of each other are never both strictly separated by a third member in the preference ranking of any voter, unless that member is also a fellow clone. Trivially, the set of all candidates makes up a clone set as does every subset consisting of one candidate. It thus makes no sense to just call a candidate a clone unless it is in the context of a clone set which contains at least two elements and is a proper subset of the set of all candidates.
It is desirable for the outcome of an election to be essentially unaffected by the addition or removal of clones. Adding or removing a clone candidate should only change the winner if the old winner, the new winner, and the candidate added or removed are all clones of each other. A voting system that satisfies this criterion is considered "independent of clones". Independence of clones was first formulated by Nicolaus Tideman.
The existence of a true clone set in a public election is improbable, as it only takes one voter to break up a clone set. As a result of this fact, some argue that the independence of clones criterion has limited relevance to real-world elections. This criterion is still used in academic analysis, however, as many voting systems behave similarly when handling both clones and closely affiliated candidates with common supporters.
Types of strategic nomination
- Vote-splitting happens when adding similar or clone candidates decreases the chance of any of them winning, also known as a spoiler effect. Methods that are vulnerable to this include the plurality voting system and two-round runoff voting.
- Teaming happens when adding more candidates actually helps the chances of any of them winning, as can occur in Borda counts.
- Crowding happens when adding candidates affects the outcome of an election without either helping or harming the chances of their factional group, but instead affecting another group. This can occur in Copeland's method.
- T. Nicolaus Tideman, "Independence of clones as a criterion for voting rules", Social Choice and Welfare Vol. 4, No. 3 (1987), pp. 185–206.
- Independence from similar alternatives Article by Stephen Eppley