AbstractOne of risk measures’ key purposes is to consistently rank and distinguish between different risk
profiles. From a practical perspective, a risk measure should also be robust, that is, insensitive to small perturbations
in input assumptions. It is known in the literature [14, 39], that strong assumptions on the risk
measure’s ability to distinguish between risks may lead to a lack of robustness. We address the trade-off between
robustness and consistent risk ranking by specifying the regions in the space of distribution functions,
where law-invariant convex risk measures are indeed robust. Examples include the set of random variables
with bounded second moment and those that are less volatile (in convex order) than random variables in a
given uniformly integrable set. Typically, a risk measure is evaluated on the output of an aggregation function
defined on a set of random input vectors. Extending the definition of robustness to this setting, we find
that law-invariant convex risk measures are robust for any aggregation function that satisfies a linear growth
condition in the tail, provided that the set of possible marginals is uniformly integrable. Thus, we obtain that
all law-invariant convex risk measures possess the aggregation-robustness property introduced by [26] and
further studied by [40]. This is in contrast to the widely-used, non-convex, risk measure Value-at-Risk, whose
robustness in a risk aggregation context requires restricting the possible dependence structures of the input
vectors.