Judgment aggregation without full rationality

Several recent results on the aggregation of judgments over logically connected propositions show that, under certain conditions, dictatorships are the only independent (i.e., propositionwise) aggregation rules generating fully rational (i.e., complete and consistent) collective judgments. A frequently mentioned route to avoid dictatorships is to allow incomplete collective judgments. We show that this route does not lead very far: we obtain (strong) oligarchies rather than dictatorships if instead of full rationality we merely require that collective judgments be deductively closed, arguably a minimal condition of rationality (compatible even with empty judgment sets). We derive several characterizations of oligarchies and provide illustrative applications to Arrowian preference aggregation and Kasher and Rubinstein''s group identification problem.mathematical economics;

Judgment aggregation without full rationality

Several recent results on the aggregation of judgments over logically connected propositions show that, under certain conditions, dictatorships are the only propositionwise aggregation functions generating fully rational (i.e., complete and consistent) collective judgments. A frequently mentioned route to avoid dictatorships is to allow incomplete collective judgments. We show that this route does not lead very far: we obtain oligarchies rather than dictatorships if instead of full rationality we merely require that collective judgments be deductively closed, arguably a minimal condition of rationality, compatible even with empty judgment sets. We derive several characterizations of oligarchies and provide illustrative applications to Arrowian preference aggregation and Kasher and Rubinstein''s group identification problem.mathematical economics;

Overconfidence in Photometric Redshift Estimation

We describe a new test of photometric redshift performance given a spectroscopic redshift sample. This test complements the traditional comparison of redshift {\it differences} by testing whether the probability density functions $p(z)$ have the correct {\it width}. We test two photometric redshift codes, BPZ and EAZY, on each of two data sets and find that BPZ is consistently overconfident (the $p(z)$ are too narrow) while EAZY produces approximately the correct level of confidence. We show that this is because EAZY models the uncertainty in its spectral energy distribution templates, and that post-hoc smoothing of the BPZ $p(z)$ provides a reasonable substitute for detailed modeling of template uncertainties. Either remedy still leaves a small surplus of galaxies with spectroscopic redshift very far from the peaks. Thus, better modeling of low-probability tails will be needed for high-precision work such as dark energy constraints with the Large Synoptic Survey Telescope and other large surveys.Comment: accepted to MNRA

Impossibilities of Paretian Social Welfare Functions for Infinite Utility Streams with Distributive Equity

This paper examines the logical relationship between distributive equity and efficiency in aggregating infinite utility streams. Our main results show that there exist social welfare functions which satisfy the axioms of Pigou-Dalton Transfer Principle and a weak version of efficiency, but there exists no social welfare function which satisfies all of the distributive equity requirements and Weak Pareto Principle at the same time. Thus, we can prove that no Paretian ranking can satisfy the numerical representability and all of the distributive equity properties in the setting of intertemporal social choice.

A liberal paradox for judgment aggregation

In the emerging literature on judgment (as opposed to preference) aggregation, expert rights or liberal rights have not been investigated yet. When a group forms collective beliefs, it may assign experts with special knowledge on certain propositions the right to determine the collective judgment on those propositions; and, when a group forms collective goals or desires, it may assign individuals specially affected by certain propositions similar rights on those propositions. We identify a problem similar to, but more general than, Sen's `liberal paradox': Under plausible conditions, the assignment of such rights to two or more individuals (or subgroups) is inconsistent with the unanimity principle, whereby propositions accepted by all individuals must be collectively accepted. So a group respecting expert or liberal rights on certain propositions must sometimes overrule its unanimous judgments on others. The inconsistency does not arise if either different individuals' rights are `disconnected' or individuals are `agnostic/tolerant' or `deferring/empathetic' towards other individuals' rights. Our findings have implications for the design of mechanisms by which groups (societies, committees, expert panels, organizations) can reach decisions on systems of interconnected propositions.liberal paradox, liberal right, expert right, subgroup rights, unanimity principle, judgment aggregation, empathy, deferral, tolerance, agnosticism

Complex Collective Decisions and the Probability of Collective Inconsistencies

Many groups are required to make collective decisions over multiple interconnected propositions. The "doctrinal paradox" or "discursive dilemma" shows that propostionwise majority voting can lead to inconsistent collective outcomes, even when the judgments of individual group members are consistent. How likely is the occurence of this paradox? This paper develops a simple model for determining the probability of the paradox's occurrence, given various assumptions about the probability of different individual judgments. Several convergence results will be proved, identifying conditions under which the probability of the paradox's occurrence converges to certainty as the number of individuals increases, and conditions under which that probability vanishes. The present model will also be used for assessing the "truth-tracking" performance of two escape-routes from the paradox, the premise- and conclusion-based procedures. Finally, the results on the probability of the doctrinal paradox will be compared with existing results on the probability of Condorcet's paradox of cyclical preferences. It will be suggested that the doctrinal paradox is more likely to occur than Condorcet's paradox.

Judgment aggregation by quota rules

It is known that majority voting among several individuals on logically interconnected propositions may generate irrational collective judgments. We generalize majority voting by considering quota rules, which accept each proposition if and only if the number of individuals accepting it exceeds some (proposition-specific) threshold. After characterizing quota rules, we prove necessary and sufficient conditions under which their outcomes satisfy various rationality conditions. We also consider sequential quota rules, which adjudicate propositions sequentially, letting earlier judgments constrain later ones. While ensuring rationality, sequential rules may be path-dependent. We characterize path-independence and prove its equivalence to strategy- proofness under mild conditions. Our results generalize earlier (im)possibility theorems.Judgment aggregation, quota rules, collective rationality, path-dependence, strategy-proofness, formal logic

Aggregation theory and the relevance of some issues to others

I propose a general collective decision problem consisting in many issues that are interconnected in two ways: by mutual constraints and by connections of relevance. Aggregate decisions should respect the mutual constraints, and be based on relevant information only. This general informational constraint has many special cases, including premise-basedness and Arrow''s independence condition; they result from special notions of relevance. The existence and nature of (non-degenerate) aggregation rules depends on both types of connections. One result, if applied to the preference aggregation problem and adopting Arrow''s notion of (ir)relevance, becomes Arrow''s Theorem, without excluding indifferences unlike in earlier generalisations.mathematical economics;