ILLC Workshop on Collective Decision Making 2015

Thursday AM (Room F0.01, Chair: Ulle Endriss)
9:30-10:15Stéphane Airiau
 Voting in Parallel Universes
10:15-11:00Hans Peters
 Choosing k from m: Feasible Elimination Procedures Reconsidered
11:00-11:30coffee break
11:30-12:15Ronald de Haan
 What Complexity Theory Can Tell us about Judgment Aggregation
Friday AM (Room A0.08, Chair Stéphane Airiau)
10:15-11:00Umberto Grandi
 Propositional Opinion Diffusion
11:00-11:30coffee break
11:30-12:15Barbara Vis
 The Output of Aggregated Preferences: Why Government Parties Change their Platform more than Opposition Parties
Thursday PM (Room F0.01, Chair: Olivier Cailloux)
14:30-15:15Constanze Binder
 Walking a Mile in your Shoes: An Escape from Arrovian Impossibilities
15:15-16:00Sujata Ghosh
 We Discuss, then we Decide: Reliability-based Preference Change
16:00-16:30coffee break
16:30-17:15Lora Aroyo
 CrowdTruth: Crowdsourcing Approach to Harnessing Diversity of Human Interpretations
Friday PM (Room A0.08, Chair: Umberto Grandi)
14:30-15:15Vahid Hashemi
 Measuring Diversity of Preferences
15:15-16:00Olivier Cailloux
 Arguing about Voting Rules
16:00-16:30coffee break
16:30-17:15Giovanni Cinà
 Arrow's Theorem in Modal Logic

Speaker: Stéphane Airiau (LAMSADE, Université Paris-Dauphine)
Title: Voting in Parallel Universes (slides)

Abstract: We want to study a family of voting rules based on a pair (v,σ) where v is a known voting rule and σ is a linear order that ensures that (v,σ) is resolute. The three rules we consider are (1) the "union rule" in which the set of winners is the union of the winners of (v,σ) over all linear orders, (2) a randomized rule that computes the proportion of linear orders for which a given candidate is the winner of (v,σ), and (3) the "argmax rule" that selects the candidates that win most often over all linear orders. In this talk, we will focus on STV (in which case the union rule is known as "parallel universes") and on a polynomial rule that selects a Banks winner (in which case the union rule is the Banks rule). We will discuss complexity issues as well as simulation results. (This is joint work with Justin Kruger and Jérôme Lang.)

Speaker: Lora Aroyo (Computer Science, VU Amsterdam)
Title: CrowdTruth: Crowdsourcing Approach to Harnessing Diversity of Human Interpretations

Abstract: Human annotation (or semantic interpretation) is a critical part of big data semantics, but it is based on an antiquated ideal of a single correct truth. This presentation exposes seven myths about human annotation and dispels the myth of a single truth with examples from our research. A new theory of truth, CrowdTruth, is based on subjective human interpretation on the same objects (in our examples, sentences) across a crowd, providing a representation of subjectivity and a range of reasonable interpretations. CrowdTruth has allowed us to identify the myths of human annotation and paint a more accurate picture of human performance of semantic interpretation for machines to attain. Implementation of the CrowdTruth framework and some further reading can be found at

Speaker: Constanze Binder (Philosophy, Erasmus University Rotterdam)
Title: Walking a Mile in your Shoes: An Escape from Arrovian Impossibilities (slides)

Abstract: This paper explores approaches to comparative justice (Sen, 2009) by drawing on Social Choice Theory. We introduce a procedure to correct for the influence of unquestioned parochial values on individual justice rankings: individuals are put into the position of other members of society allowing them to question (and possibly change) their justice ordering. In a first step, it is shown under which conditions this procedure leads to a domain restriction such that majority rule yields a social justice ordering. In a second step, it is examined how the introduced procedure can be used to distinguish between "reasoned" and "unreasoned" consensus. The paper concludes with a discussion as to how the findings cast doubt on the unqualified acceptance of the (weak) Pareto condition.

Speaker: Olivier Caillloux (HEUDIASYC, Université de Technologie de Compiègne)
Title: Arguing about Voting Rules (slides)

Abstract: When the members of a group have to make a decision, they can use a voting rule to aggregate their preferences. Different voting rules have different properties, and social choice theorists have found arguments for and against most of them. These arguments are aimed at the sophisticated reader, used to mathematical formalism. We propose a logic-based language to instantiate such arguments in concrete terms in order to help individuals understand the strengths and weaknesses of different voting rules. Our approach permits to automatically derive a justification for a given election outcome or to support a group in arguing over which voting rule to use. We exemplify our approach with an in-depth study of the Borda rule. (This is joint work with Ulle Endriss.)

Speaker: Giovanni Cinà (ILLC, University of Amsterdam)
Title: Arrow's Theorem in Modal Logic (slides)

Abstract: We show how to formalise Arrow's Theorem on the impossibility of devising a method for preference aggregation that is both independent of irrelevant alternatives and Pareto effcient by using a modal logic of social choice functions. We also provide a syntactic proof of the theorem in that logic. While prior work has been successful in applying tools from logic and automated reasoning to social choice theory, this is the first human-readable formalisation of the Arrovian framework allowing for a direct derivation of the theorem. (This is joint work with Ulle Endriss.)

Speaker: Sujata Ghosh (Indian Statistical Institute, Chennai)
Title: We Discuss, then we Decide: Reliability-based Preference Change (slides)

Abstract: This work models situations akin to public deliberation leading to preference changes. A set of agents is considered, each endowed with a preference relation over a set of objects and a reliability relation over the involved agents. Different ways in which the public announcement of the current individual preferences can influence the agents' future preferences are studied. Special emphasis is given to ways in which the repetitive public announcements of the individual preferences lead to unanimity on preferences. (This is joint work with Fernando R. Velázquez-Quesada.)

Speaker: Umberto Grandi (IRIT & Université Toulouse 1 Capitole)
Title: Propositional Opinion Diffusion (slides)

Abstract: We present a formal model of opinion diffusion and formation which combines notions from social network analysis together with concepts and techniques from judgment aggregation and merging. The model allows us to study the propagation of individual opinions, represented in the form of yes/no answers to a set of multiple binary issues, in a multiagent system linked by an influence network. The process is iterative with discrete time. We are interested in characterizing properties of the network structure which guarantee convergence of the iterative process for every initial configuration of the agents’ opinions, and in developing tractable algorithms for computing the set of opinions at convergence. (This is joint work with Emiliano Lorini and Laurent Perrussel.)

Speaker: Ronald de Haan (Computer Science, TU Vienna)
Title: What Complexity Theory Can Tell us about Judgment Aggregation (slides)

Abstract: The interdisciplinary area of judgment aggregation studies the question of how a group of individuals can combine their individual opinions into a collective opinion. There are several possible (formal) languages that can be used to specify opinions, and there are plenty of aggregation procedures that can be used to form the group opinion. One aim of judgment aggregation is to discern various desirable properties of the different variants, which can help guide choosing between options. One type of such properties, stemming from theoretical computer science, concerns computational complexity aspects. In a nutshell, these properties are related to the question whether these aggregation methods could be used efficiently in practice. We discuss several computational complexity properties that can be used to differentiate between various specification languages and various aggregation procedures. Moreover, we reflect on what role these complexity considerations could take in future research on judgment aggregation.

Speaker: Vahid Hashemi (ILLC, University of Amsterdam)
Title: Measuring Diversity of Preferences (

Abstract: Measuring the diversity of a set of preferences can be useful in many ways. For example, one might want to see whether a society is diverse or not, rather than aggregating the preferences; or it can tell us something about the structure of the group. While the least diverse situation is indisputably a unanimous profile, one can reasonably consider different profiles as the most diverse. Likewise, there can be different approaches to measure diversity. We introduce a general framework for measuring the degree of diversity in the preferences held by the members of a group. We formalize and investigate three specific approaches within that framework. We analyze their differences and properties, using both the axiomatic method and empirical studies. (This is joint work with Ulle Endriss.)

Speaker: Hans Peters (Buisness and Economics, Maastricht University)
Title: Choosing k from m: Feasible Elimination Procedures Reconsidered (slides)

Abstract: We show that feasible elimination procedures (Peleg, 1978) can be used to select k from m alternatives. An important advantage of this method is the core property: no coalition can guarantee an outcome that is preferred by all its members. We also provide an axiomatic characterization for the case k = 1, using the conditions of anonymity, Maskin monotonicity, and independent blocking. Finally, we show for any k that outcomes of feasible elimination procedures can be computed in polynomial time, by showing that the problem is computationally equivalent to finding a maximal matching in a bipartite graph. (Based on joint work with Bezalel Peleg.)

Speaker: Daniele Porello (Institute of Cognitive Science and Technology, CNR Trento)
Title: The Index Problem of the Difference Principle [talk cancelled]

Abstract: The Rawlsian difference principle is one of the main foundations of egalitarian distributive justice. In its conceptual formulation, it applies to the distribution primary goods, that are all-purpose means that enable agents to pursue their life plans. The difference principle can be summarized by saying that an allocation should maximize the amounts of primary goods allocated to the worst-off. Since primary goods are heterogeneous, the problem of finding an index that compares bundles of primary goods is compelling for a precise definition of the difference principle. In this talk, we discuss the debate concerning the formalizations of an index of primary goods, we focus on the formal solution of the indexing problem provided by Fleurbaey, and we suggest an alternative formalization that better copes with the Rawlsian view of distributive justice.

Speaker: Barbara Vis (Political Science and Public Administration, VU Amsterdam)
Title: The Output of Aggregated Preferences: Why Government Parties Change their Platform more than Opposition Parties (slides)

Abstract: Considerable ambiguity exists regarding the effect of government/opposition status on parties' decision to change their platform. Existing theories predict that: (1) it has no effect, (2) opposition parties change more, (3) opposition parties change more after several spells in opposition and (4) parties' responses vary because of different goal orientations. We propose that a party’s aspiration to office, measured by its historical success or failure in entering office, determines its reaction to being in opposition or government. We hypothesize that, because of loss aversion, parties with low office aspiration change more when they are in office than when they are in opposition. Conversely, parties with high office aspiration change more as opposition party than as government party. We find evidence for these hypotheses through a pooled time-series cross sectional analysis of 1,686 platform changes in 21 democracies, using the Comparative Manifesto Data and an innovative measure of party platform change. (Based on joint work with Gijs Schumacher, Marc van de Wardt and Michael Baggesen Klitgaard.)