Selfishness is behavior that is directed at furthering one's own interests, without regard to, or even at the expense of, others'. A selfish person might, for example, eat all of his cookies rather than offering one to a friend, if he has no reason to believe that friend will reciprocate. Economists generally distinguish between selfishness and rational self-interest. A person can engage in unselfish behavior out of rational self-interest because he believes he can gain greater satisfaction from devoting a given resource to improving others' circumstances than to improving his own circumstances.
Mises, in Human Action