However, the theory of "psychological egoism" is meaningless. It says nothing, because in no possible Universe would it not be true. It is impossible to have a psychology whether man or beast or sentient alien that isn't self-interested because anything a person is interested in is an interest that belongs to their self.
What's worse is you wind up with the suggestion that there is right way to be "self-interested" or that there is such a thing as "Rational self-interest". Reason is only useful for figuring out means. Ends are entirely subjective. Hence we should embrace self-interest with no expectations that it be rational, with no boundaries for what a self-interest may entail.