Choosing How to Choose: Self Stable Majority Rules

We consider the endogenous choice of a voting rule, characterized by the majority size needed to elect change over the status quo, by a society who will use the rule to make future decisions. Under simple assumptions on the uncertainty concerning the future alternatives that will be voted upon, voters' have induced preferences over voting rules that are single-peaked and intermediate. We explore the existence of self-stable voting rules, i.e., voting rules such that there is no alternative rule that would beat the given voting rule if the given voting rule is used to choose between the rules. There are situations where self-stable voting rules do not exist. We explore conditions that guarantee existence, as well as issues relating to efficiency and constitutional design.

On the Weights of Nations: Assigning Voting Weights in a Heterogeneous Union

Consider a voting procedure where countries, states, or districts comprising a union each elect representatives who then participate in later votes at the union level on their behalf. The countries, provinces, and states may vary in their populations and composition. If we wish to maximize the total expected utility of all agents in the union, how to weight the votes of the representatives of the different countries, states or districts at the union level? We provide a simple characterization of the efficient voting rule in terms of the weights assigned to different districts and the voting threshold (how large a qualified majority is needed to induce change versus the status quo). Next, in the context of a model of the correlation structure of agents preferences, we analyze how voting weights relate to the population size of a country. We then analyze the voting weights in Council of the European Union under the Nice Treaty and the recently proposed constitution, and contrast them under different versions of our model, and compare them to the weights derived from poll data.Majority rule, Voting, Weighted voting, European Union

Self-Selection Consistent Choices

This paper studies collective choice rules whose outcomes consist of a collection of simultaneous decisions, each one of which is the only concern of some group of individuals in society. The need for such rules arises in different contexts, including the establishment of jurisdictions, the location of multiple public facilities, or the election of representative committees. We define a notion of allocation consistency requiring that each partial aspect of the global decision taken by society as a whole should be ratified by the group of agents who are directly concerned with this particular aspect. We investigate the possibility of designing allocation consistent rules which satisfy the Condorcet criterion and respect different notions of voluntarism.

Voting by Committees Under Constraints

Many problems of social choice take the following form. There are n voters and a set of k objects. These objects may be bills considered by a legislature, candidates to some set of positions, or the collection of characteristics which distinguish a social alternative from another. The voters must choose a subset of the set of objects. Sometimes, any combination of objects is feasible: for example, if we consider the election of candidates to join a club which is ready to admit as many of them as the voters choose. It is for these cases that Barbera, Sonnenschein, and Zhou (1991) provided characterizations of all voting procedures which are strategy-proof and respect voter's sovereignty when voters' preferences are additively representable, and also when these are separable. For both of these restricted domains, voting by committees turns out to be the family of all rules satisfying the above requirements. Most often, though, some combinations of objects are not feasible, while others are: if there are more candidates than positions to be filled, only sets of size less than or equal to the available number of slots are feasible. Our purpose in this paper is to characterize the families of strategy-proof voting procedures when not all possible subsets of objects are feasible, and voters' preferences are separable or additively representable. Our main conclusions are the following. First: all rules that satisfy strategy-proofness must still be voting by committees. Second: the committees for different objects must be interrelated, in precise ways which depend on what families of sets of objects are feasible. Third: unlike in Barbera, Sonnenschein, and Zhou (1991), the class of strategy- proof rules when preferences are additively representable can be substantially larger that the set of rules satisfying the same requirement when voter' preferences are separable.

Strategy-Proof Allotment Rules

We consider the problem of alloting shares of a task or good among agents with single peaked preferences over their own shares. Previous characterizations have examined rules, such as the uniform rule, which satisfy various symmetry requirements. We consider the case where agents might begin with natural claims to minimal or maximal allotments, or might be treated with different priorities. We provide characterizations of the rules which are strategy-proof and efficient, but may treat individuals asymmetrically.

A Characterization of Strategy-Proof Social Choice Functions for Economies with Pure Public Goods

We characterize strategy-proof social choice functions when individual have strictly quasi-concave, continuous and satiated utility functions on convex subsets |R^l, representing preferences for the provision of l pure public goods. When specialized to the case l=1, these assumptions amount to requiring that preferences are single peaked, and for such a domain there exists a wide class of strategy-proof social choice functions. These were studied by Moulin (1980) under additional assumptions. Our first results characterize the complete class, after an appropriate extension of the single-peakedness condition. The new characterization retains the flavor of Moulin's elegant representation theorem. For the general l-dimensional case, previous results have shown that there is no efficient, strategy-proof, nondictatorial social choice function, even within the domain restrictions under consideration [Border and Jordan (1983), Zhou (1991)]. In fact, Zhou's powerful results indicates that nondictatorial strategy-proof s.c.f.'s will have a range of dimension one. This allows us to conclude with a complete characterization of all strategy-proof s.c.f.'s on |R^l, because restrictions of preferences from our admissible class to one dimensional subsets satisfy the slightly generalized notion of single-peakedness that is used in our characterization for the case l=1. We feel that a complete knowledge of the class of strategy-proof mechanisms, in this as well as in other contexts, is an important step in the analysis of the trade-offs between strategy-proofness and other performance criteria, like efficiency.

Strategy-Proof Exchange

We consider the allocation of goods in exchange economies with a finite number of agents who may have private information about their preferences. In such a setting, standard allocation rules such as Walrasian equilibria or rational expectations equilibria are not compatible with individual incentives. We characterize the set of allocations rules which are incentive compatible, or in other words, the set of strategy-proof social choice functions. The social choice functions which are strategy-proof are those which can be obtained from trading according to pre-specified proportions. The number of proportions which can be accommodated is proportional to the number of agents. Such rules are necessarily inefficient, even in the limit as the economy grows.

Sub one per cent mass fractions of young stars in red massive galaxies

Early-type galaxies are considered to be the end products of massive galaxy formation1. Optical spectroscopic studies reveal that massive early-type galaxies formed the bulk of their stars over short timescales (≲≲1 Gyr) and at high redshift (z ≳≳ 2), followed by passive evolution to the present2. However, their optical spectra are unable to constrain small episodes of recent star formation, since they are dominated by old stars. Fortunately, this problem can be tackled in the ultraviolet range. While recent studies that make use of ultraviolet absorption lines have suggested the presence of young stars in a few early-type galaxies3, the age and mass fractions of young stars and their dependence on galaxy mass are unknown. Here we report a detailed study of these young stellar populations, from high-quality stacked spectra of 28,663 galaxies from the BOSS survey4, analysing optical and ultraviolet absorption lines simultaneously. We find that residual star formation is ubiquitous in massive early-type galaxies, measuring average mass fractions of 0.5% in young stars in the last 2 Gyr of their evolution. This fraction shows a decreasing trend with galaxy stellar mass, consistent with a downsizing scenario5. We also find that synthetic galaxies from state-of-the-art cosmological numerical simulations6 substantially overproduce both intermediate and young stellar populations. Therefore, our results pose stringent constraints on numerical simulations of galaxy formation6,7