Strategic voting and nomination

Using computer simulations based on three separate data generating processes, I estimate the fraction of elections in which sincere voting will be a core equilibrium given each of eight single-winner voting rules. Additionally, I determine how often each voting rule is vulnerable to simple voting strategies such as 'burying' and 'compromising', and how often each voting rule gives an incentive for non-winning candidates to enter or leave races. I find that Hare is least vulnerable to strategic voting in general, whereas Borda, Coombs, approval, and range are most vulnerable. I find that plurality is most vulnerable to compromising and strategic exit (which can both reinforce two-party systems), and that Borda is most vulnerable to strategic entry. I support my key results with analytical proofs.strategic voting; tactical voting; strategic nomination; Condorcet; alternative vote; Borda count; approval voting

A Smooth Transition from Powerlessness to Absolute Power

We study the phase transition of the coalitional manipulation problem for generalized scoring rules. Previously it has been shown that, under some conditions on the distribution of votes, if the number of manipulators is $o(\sqrt{n})$, where $n$ is the number of voters, then the probability that a random profile is manipulable by the coalition goes to zero as the number of voters goes to infinity, whereas if the number of manipulators is $\omega(\sqrt{n})$, then the probability that a random profile is manipulable goes to one. Here we consider the critical window, where a coalition has size $c\sqrt{n}$, and we show that as $c$ goes from zero to infinity, the limiting probability that a random profile is manipulable goes from zero to one in a smooth fashion, i.e., there is a smooth phase transition between the two regimes. This result analytically validates recent empirical results, and suggests that deciding the coalitional manipulation problem may be of limited computational hardness in practice.Comment: 22 pages; v2 contains minor changes and corrections; v3 contains minor changes after comments of reviewer

Dictatorship versus manipulability

The Gibbard-Satterthwaite (1973/75) theorem roughly states that we have to accept dictatorship or manipulability in case of at least three alternatives. A large strand of the literature estimates the degree of manipulability of social choice functions (e.g. Aleskerov and Kurbanov, 1999, Favardin et al., 2002, and Aleskerov et al., 2012), most of them employing the Nitzan-Kelly index of manipulability. We take a different approach and introduce a non-dictatorship index based on our recent work (Bednay et al., 2017), where we have analysed social choice functions based on their distances to the dictatorial rules. By employing computer simulations, we investigate the relationship between the manipulability and nondictatorship indices of some prominent social choice functions, putting them into a common framework

Probabilistic Evaluation of Preference Aggregation Functions: A Statistical Approach in Social Choice Theory

A statistical criterion for evaluating the appropriateness of preference aggregation functions for a fixed group of persons is introduced. Specifically, we propose a method comparing aggregation procedures by relying on probabilistic information on the homogeneity structure of the group members’ preferences. For utilizing the available information, we give a minimal axiomatization as well as a proposal for measuring homogeneity and discuss related work. Based on our measure, the group specific probability governing the constitution of preference profiles is approximated, either relying on maximum entropy or imprecise probabilities. Finally, we investigate our framework by comparing aggregation rules in a small study

Aggregation of Rankings in Figure Skating

We scrutinize and compare, from the perspective of modern theory of social choice, two rules that have been used to rank competitors in Figure Skating for the past decades. The first rule has been in use at least from 1982 until 1998, when it was replaced by a new one. We also compare these two rules with the Borda and the Kemeny rules. The four rules are illustrated with examples and with the data of 30 Olympic competitions. The comparisons show that the choice of a rule can have a real impact on the rankings. In these data, we found as many as 19 cycles of the majority relation, involving as many as nine skaters. In this context, the Kemeny rule appears as a natural extension of the Condorcet rule. As a side result, we show that the Copeland rule can be used to partition the skaters in such a way that it suffice to find Kemeny rankings within subsets of the partition that are not singletons and then, to juxtapose these rankings to get a complete Kemeny ranking. We also propose the concept of the mean Kemeny ranking, which when it exists, may obviate the multiplicity of Kemeny rankings. Finally, the fours rules are examined in terms of their manipulability. It appears that the new rule used in Figure Skating may be more difficult to manipulate than the previous one but less so than the Kemeny rule.Figure skating, ranking rules, vote aggregation, cycles, maximum likelihood, Kemeny, Copeland, Borda, manipulation

Welfare effects of strategic voting under scoring rules

Strategic voting, or manipulation, is the process by which a voter misrepresents his preferences in an attempt to elect an outcome that he considers preferable to the outcome under sincere voting. It is generally agreed that manipulation is a negative feature of elections, and much effort has been spent on gauging the vulnerability of voting rules to manipulation. However, the question of why manipulation is actually bad is less commonly asked. One way to measure the effect of manipulation on an outcome is by comparing a numeric measure of social welfare under sincere behaviour to that in the presence of a manipulator. In this paper we conduct numeric experiments to assess the effects of manipulation on social welfare under scoring rules. We find that manipulation is usually negative, and in most cases the optimum rule with a manipulator is different to the one with sincere voters