2026/03/02 by Christopher G. Lamoureux · 1 voice
Economics, Econometrics and Finance · #q-fin.PM
arxiv published 2026/03/02 · arxiv updated 2026/03/06
Parametric portfolio policies may experience estimation risk. I develop a generalized Bayesian framework that updates priors, delivering a posterior distribution over characteristic tilts and out-of-sample returns that is the unique belief-updating rule consistent with the investor's utility function, requiring no model for the return generating process. The Gibbs posterior is the closest distribution to the prior in Kullback-Leibler divergence subject to utility maximization. The posterior's scaling parameter λ controls the weight placed on data relative to the prior. I develop a KNEEDLE algorithm to select optimal λ^* in-sample by trading off posterior precision against numerical fragility, eliminating the need for out-of-sample validation. I apply this to U.S. equities (1955-2024), and confirm characteristic-based gains concentrate pre-2000. I find that λ^* varies meaningfully with risk aversion and depends on higher-order moments.