Top-Pair and Top-Triple Monotonicity (original) (raw)

Abstract

We say that a social choice function (SCF) satisfies Top-k Monotonicity if the following holds. Suppose the outcome of the SCF at a preference profile is one of the top _k_-ranked alternatives for voter i. Let the set of these k alternatives be denoted by B. Suppose that _i_’s preference ordering changes in such a way that the set of first _k_-ranked alternatives remains the set B. Then the outcome at the new profile must belong to B. This definition of monotonicity arises naturally from considerations of set “improvements” and is weaker than the axioms of strong positive association and Maskin Monotonicity. Our main results are that if there are two voters then a SCF satisfies unanimity and Top-2 or Top-pair Monotonicity if and only if it is dictatorial. If there are more than two voters, then Top-pair Monotonicity must be replaced by Top-3 Monotonicity (or Top-triple Monotonicity) for the analogous result. Our results demonstrate that connection between dictatorship and “improvement” axioms is stronger than that suggested by the Muller–Satterthwaite result (Muller and Satterthwaite in J Econ Theory 14:412–418, 1977) and the Gibbard–Sattherthwaite theorem.

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Authors and Affiliations

  1. Concordia University, Montreal, Canada
    Dipjyoti Majumdar
  2. Indian Statistical Institute, New Delhi, India
    Arunava Sen

Authors

  1. Dipjyoti Majumdar
  2. Arunava Sen

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Correspondence toArunava Sen.

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Majumdar, D., Sen, A. Top-Pair and Top-Triple Monotonicity.Soc Choice Welfare 27, 175–187 (2006). https://doi.org/10.1007/s00355-006-0119-x

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