 Providing definitions for “egoism” and “altruism” is a contentious matter, since these terms have been understood in radically different ways both in philosophy and in the biological and social sciences. In this entry the focus will be on the most widespread interpretation of “egoism” and “altruism”, understood as descriptive claims about human psychology, within philosophy. We’ll call it “the standard account”, versions of which have been offered by numerous authors including Broad (1950), Feinberg (1965 [1999]), Sober and Wilson (1998: Chs. 6 &amp; 7), Rachels (2003: Ch. 6), Joyce (2006: Ch. 1), Kitcher (2010, 2011: Ch. 1), May (2011a), and many others. Not surprisingly, there are minor differences among the accounts provided by these authors, and those differences occasionally provoke disagreement in the literature. But all of them bear a strong family resemblance to the one we’re about to  sketch.[5] 