We consider collective decisions under uncertainty, in which different agents may
have not only different beliefs, but also different ambiguity attitudes --in particular,
they may or may not be subjective expected utility maximizers. We assume that
the space of possible states of nature is a Polish space. We consider sequences of
acts which are "almost-objectively uncertain" in the sense that asymptotically, all
agents almost-agree about the probabilities of the underlying events. We impose a
weak ex ante Pareto axiom which applies only to asymptotic preferences along such
almost-objective sequences. We show that this axiom implies that the social welfare
function is utilitarian (i.e. a weighted sum of individual utility functions). But it
does not impose any relationship between individual and collective beliefs, or between
individual and collective ambiguity attitudes.