Impartial Evaluation under Ambiguity

How should an impartial social observer judge distributions of well-being across different individuals when there is uncertainty regarding the state world? I explore this question by imposing very weak conditions rationality and benevolent sympathy on betterness judgments under uncertainty. Although enough to be consistent with all main theories rationality, these prove sufficient rule out any heterogeneity in what good for individuals, require a neutral attitude part observer, that both individual strongly separable.