Psychological hedonism is the theory that all human choice is motivated by a desire for pleasure (or an aversion to pain).
This theory of human motivation cannot rule out altruism without being contradicted by evidence. Rather, it claims that when sane people choose to help others, it is because of the pleasure they themselves obtain, directly or indirectly, from doing so. Pyschological hedonism is a special case of psychological egoism, in which the concept of "self interest" is equated with pleasure.
See also: hedonism