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Electoral Reform and Social Choice Theory: Piecemeal Engineering and Selective Memory

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Book cover Transactions on Computational Collective Intelligence XXXIV

Part of the book series: Lecture Notes in Computer Science ((TCCI,volume 11890))

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Abstract

Most electoral reforms are dictated by recognized problems discovered in the existing procedures or - perhaps more often - by an attempt to consolidate power distributions. Very rarely, if ever, is the motivation derived from the social choice theory even though it deals with issues pertaining to what is possible and what is impossible to achieve by using given procedures in general. We discuss some reforms focusing particularly on a relatively recent one proposed by Eric Maskin and Amartya Sen. It differs from many of its predecessors in invoking social choice considerations in proposing a new system of electing representatives. At the same time it exemplifies the tradeoffs involved in abandoning existing systems and adopting new ones.

The author thanks the referees for numerous constructive comments.

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Notes

  1. 1.

    It is relatively straightforward to see how this conclusion is derived. To wit, suppose that there is a Condorcet loser, say x, in a profile consisting of n voters and k alternatives. This means that in each pairwise comparison, the maximum number of votes for x is strictly less than n/2. Hence x’s Borda score is less than \((k-1)\times (n/2)\). If all alternatives had the same or smaller Borda score than x (which would make x the Borda winner), the total number of Borda scores would be no larger than: \(k\times (k-1)\times n/2 \). Now this upper bound is strictly less than the sum total of Borda scores in any profile, viz. \( n \times (k^{2} - k)/2 = k \times (k-1) \times n/2\). (The number of pairwise comparisons involving different alternatives is \(k^2 - k\) with the sum of entry (ij) and entry (ji) being equal to the number of voters, n, for all alternatives i and j.) Therefore, in any profile there must be at least one alternative with a strictly larger Borda score than that of the Condorcet loser. Hence, the latter cannot be elected by the Borda count.

  2. 2.

    A strong Condorcet winner is an alternative ranked first by more than half of the electorate. Obviously, all procedures that elect a Condorcet winner also elect a strong Condorcet winner. The converse is not true, that is, there are procedures (e.g. plurality voting) that elect the strong Condorcet winner, but not necessarily a Condorcet winner.

  3. 3.

    Nanson’s argument to that effect is pretty similar to the one in footnote 1. It amounts to showing that the lower bound of the Borda score of a Condorcet winner is strictly larger than the average of the Borda scores.

  4. 4.

    The differences and similarities of Nanson’s and Baldwin’s rules are discussed in [7, 14, 15].

  5. 5.

    For example, let a three-person four-alternative profile be the following: 1 voter: ABCD, 1 voter: CBDA, 1 voter: DBAC. Here B is the Condorcet winner.

  6. 6.

    Moulin’s lower bound for the number of voters was 25. This bound has more recently been lowered to 12 by Brandt et al. [3]. A stronger variant of Moulin’s result has subsequently been proven by Pérez [19].

  7. 7.

    Table 8 also illustrates the vulnerability to the strong no-show paradox of several other Condorcet consistent procedures: Baldwin, Black, Copeland and Kemeny. For further discussion, see [8].

  8. 8.

    For a related discussion on the MS procedure, see [18].

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Nurmi, H. (2019). Electoral Reform and Social Choice Theory: Piecemeal Engineering and Selective Memory. In: Nguyen, N., Kowalczyk, R., Mercik, J., Motylska-Kuźma, A. (eds) Transactions on Computational Collective Intelligence XXXIV. Lecture Notes in Computer Science(), vol 11890. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-60555-4_5

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