Matías Núñez

Preference Intensity Representation : Strategic Overstating in Large Elections

If voters vote strategically, is it useful to offer them the possibility of expressing nuanced opinions, or would they always overstate the intensity of their preferences? For additive voting rules, say that a ballot is extremal if it is neither abstention-like nor can be expressed as a mixture of the available ballots. We give a sufficient condition for strategic equivalence: if two rules share the same set of extremal ballots (up to an homothetic transformation), they are strategically equivalent in large elections. This condition is also necessary for the strategic equivalence of positional rules. These results do not hold for small electorates.

Approval Voting in Large Electorates

The strategic analysis of voting rules has given some insight into the understanding of their properties. However, one can assert that these analyses are “too rich” in the sense that they show that a plethora of equilibria can arise under most voting rules. In particular, there is a controversy over Approval voting or AV, a voting rule which has been called “the electoral reform of the twentieth century.” This voting rule allows the voter to vote for as many candidates as he wishes and the candidate who gets the most votes wins the election. Its detractors claim that this kind of method enhances strategic voting when compared for instance to Plurality voting (henceforth PV), whereas its proponents consider that it has several advantages as far as strategic voting is concerned. For an extensive discussion on this controversy over AV, the reader can refer to Brams (2008) and Weber (1995).

A Map of Approval Voting Equilibria Outcomes

It is commonly accepted that the multiplicity of equilibria is ubiquitous in preference aggregation games with any voting method. We prove that this mul- tiplicity is greatly reduced under some mild restrictions over social preferences when each voter can vote for as many candidates as she wishes (the Approval voting method). For scenarios with three candidates, we can hence build a map that associates any preference profile to its set of equilibria outcomes; this map is very close to the most well-known Tournament solutions.

Large Spatial Competition

We consider spatial competition when consumers are arbitrarily distributed on a compact metric space. Retailers can choose one of finitely many locations in this space. We focus on symmetric mixed equilibria which exist for any number of retailers. We prove that the distribution of retailers tends to agree with the distribution of the consumers when the number of competitors is large enough. The results are shown to be robust to the introduction of (i) randomness in the number of retailers and (ii) different ability of the retailers to attract consumers.

Electoral Thresholds as Coordination Devices

In this paper, we study one‐person–one‐vote parliamentary elections where voters care both about the winner of elections and about the composition of the parliament. Parties enter the parliament if and only if their vote share exceeds some predetermined threshold. We show that equilibria generically exist in which all parties obtain a non‐degenerate vote‐share and, perhaps more importantly, we show that the size of the electoral threshold acts as a coordination device, which crucially affects the win prospects of the Condorcet winner party. In particular, we argue that the win prospects of the Condorcet winner party decrease with the size of the entry threshold.

Revisiting the connection between the no-show paradox and monotonicity

We investigate the relation between monotonicity and the no-show paradox in voting rules. Although the literature has established their logical independence, we show, by presenting logical dependency results, that the two conditions are closer than a general logical independency result would suggest. Our analysis is made both under variable and fixed-size electorates.

Implementation Via Approval Mechanisms

We focus on the single-peaked domain and study the class of Generalized Approval Mechanisms (GAMs): First, players simultaneously select subsets of the outcome space and scores are assigned to each alternative; and, then, a given quantile of the induced score distribution is implemented. Our main finding is that essentially for every Nash-implementable welfare optimum – including the Condorcet winner alternative – there exists a GAM that Nash-implements it. Importantly, the GAM that Nash-implements the Condorcet winner alternative is the first simple simultaneous game with this feature in the literature.

Reaching consensus through approval bargaining

In the Approval Bargaining game, two players bargain over a finite set of alternatives. To this end, each one simultaneously submits a utility function u jointly with a real number α; by doing so she approves the lotteries whose expected utility according to u is at least α. The lottery to be implemented is randomly selected among the most approved ones. We first prove that there is an equilibrium where players truthfully reveal their utility function. We also show that, in any equilibrium, the equilibrium outcome is approved by both players. Finally, every equilibrium is sincere and Pareto efficient as long as both players are partially honest.

Reaching Consensus Through Simultaneous Bargaining

We propose a two-player bargaining game where each player simultaneously proposes a set of lotteries on a finite set of alternatives. If the two sets have elements in common the outcome is selected by the uniform probability measure over the intersection. If otherwise the sets do not intersect the outcome is selected by the uniform probability measure over the union. We show that this game always has an equilibrium in sincere strategies (i.e. such that players truthfully reveal their preferences). We also prove that every equilibrium is individually rational and consensual. If furthermore players are partially honest then every equilibrium is efficient and sincere. We use this result to fully characterize the set of equilibria of the game under partial honesty.

Truth-revealing voting rules for large populations *

Deterministic voting rules are notoriously susceptible to strategic voting. We propose a new solution to this problem for large electorates. For any deterministic voting rule, we can design a stochastic rule that asymptotically approximates it in the following sense: for a sufficiently large population of voters, the stochastic voting rule (i) incentivizes every voter to reveal her true preferences and (ii) produces the same outcome as the deterministic rule, with very high probability.