Psychological egoism does not prescribe how we ought to behave but rather describes how reality really is. It says that humans can only act out of self-interest and that altruism is not possible. This would seem to greatly go against the common sense view of ethics. Any action that looks like charity has to be re-interpreted as a self-interested action. There has to be an underlying motive for a person to give to charity and that motive has to be driven by self-interest.
Psychological egoism seems to be patently false. It seems, at the very least, possible for selfless actions to take place. One can think of hypothetical situations and circumstances in which altruism is possible. One example is the possibility of someone giving a large sum of money anonymously. It would seem like this person is not giving to charity to promote himself but rather out of his own heart. This one example shows that altruism is possible and thus psychological egoism is undermined.