When does aggregation reduce risk aversion?

We study the problem of risk sharing within a household facing subjective uncertainty. A household shares uncertain prospects using a social welfare function. We characterize the social welfare functions such that the household is collectively less risk averse than each member, and satisfies the Pareto principle and an independence axiom. We single out the sum of certainty equivalents as the unique member of this family which provides quasiconcave rankings over riskless allocations.