The center squeeze effect refers to a class of voting scenarios which are troublesome for many voting systems. In such a scenario, the strongest three candidates can be arranged on a spectrum such as "left", "center", and "right"; and of the three, the "center" candidate is the Condorcet winner or utilitarian winner, but loses the election.
Most consider that if the center candidate is not too far behind in honest plurality, they should be the winner, as they would beat any other candidate in a head-to-head election, and otherwise the voting system is encouraging strategy (typically, a favorite betrayal) from one of the other two groups.
(Note that "center" does not refer to an absolute political spectrum, but relative to the ideologies of the candidates. If the Libertarian Party holds an election, for instance, the winner should be near the center of Libertarian ideology, but if there are other candidates to either side, the most-representative candidate cannot win.)
The effect is not limited to 3 candidates: The more candidates there are crowding the center, the less likely they are to win.
For example, on a 2-dimensional political compass with 3 candidates, candidate B is the Condorcet winner and utilitarian winner, but is squeezed out by A and C on either side:
C would win under a single-round of FPTP, but if there is a runoff, then more of B's votes transfer to A, making A the winner:
Either way, the winner is not as good of a representative of the electorate as candidate B.
With more candidates
Center squeeze can occur under FPTP and two-round runoff with any number of candidates. If the center candidates are close enough together, honest votes will be split between all of them, electing the worst (FPTP) or second-worst (T2R) candidate.
A similar effect can occur under IRV, electing the second-worst candidate, though the effect is less extreme, since eliminated candidates transfer votes to nearby candidates, making it harder for them to be eliminated.
Systems which generally do well with center squeeze include Condorcet systems (although in some cases, a center squeeze scenario could become an opportunity for one of the wings to use burial strategy and create an artificial Condorcet cycle).
- Lewyn, Michael (2012). "Two Cheers for Instant Runoff Voting". 6 Phoenix L. Rev. Rochester, NY. 117.
third place Candidate C is a centrist who is in fact the second choice of Candidate A’s left-wing supporters and Candidate B’s right-wing supporters. ... In such a situation, Candidate C would prevail over both Candidates A ... and B ... in a one-on-one runoff election. Yet, Candidate C would not prevail under IRV because he or she finished third and thus would be the first candidate eliminated