Average utilitarianism and several related axiologies, when paired with the standard expectational theory of decision-making under risk and with reasonable empirical credences, can find their practical prescriptions overwhelmingly determined by the minuscule probability that the agent assigns to solipsism – i.e., to the hypothesis that there is only one welfare subject in the world, viz., herself.
This either (i) constitutes a reductio of these axiologies, (ii) suggests that they require bespoke decision theories, or (iii) furnishes a novel argument for ethical egoism.
Solipsism is surely improbable. But just how improbable is it? Like average
utilitarianism, it has several notable virtues: It is simple and ontologically parsimonious. It is a natural conclusion to draw from various arguments for external world skepticism (of the kind found, e.g., in Descartes (1641) and Berkeley (1710)), if one does not simultaneously accept some “rescue” hypothesis like theism. (And it is at the very least contentious whether those arguments for external world skepticism have ever been satisfactorily refuted.)
Solipsism also provides a powerful answer to the otherwise intractable question “Why am I me?”– namely, “There isn’t anyone else I could have been!”
And finally, it is a recurring and enduring idea in the history of philosophical thought, having been entertained (in various forms) by such thinkers as the Buddhist philosopher Ratnakirti (see Kajiyama (1965)), Wittgenstein (1922), and Hare (2009).
There's also some good discussion in this Dank EA Memes thread.