On a fundamental difference between Bayesian and frequentist approaches to robustness

Heavy-tailed models are often used as a way to gain robustness against outliers in Bayesian analyses. On the other side, in frequentist analyses, M-estimators are often employed. In this paper, the two approaches are reconciled by considering M-estimators as maximum likelihood estimators of heavy-tailed models. We realize that, even from this perspective, there is a fundamental difference in that frequentists do not require these heavy-tailed models to be proper. It is shown what the difference between improper and proper heavy-tailed models can be in terms of estimation results through two real-data analyses based on linear regression. The findings of this paper make us ponder on the use of improper heavy-tailed data models in Bayesian analyses, an approach which is seen to fit within the generalized Bayesian framework of Bissiri et al. (2016) when combined with proper prior distributions yielding proper (generalized) posterior distributions.