William David MacAskill (né Crouch; born 24 March 1987) is a Scottish philosopher and ethicist, who is one of the originators of the effective altruism movement. He is Associate Professor and Tutorial Fellow at Lincoln College, Oxford, Chair of the Advisor Board at the Global Priorities Institute at the University of Oxford, and Director of the Forethought Foundation for Global Priorities Research. MacAskill is also the co-founder of Giving What We Can, the Centre for Effective Altruism and 80,000 Hours. He is the author of the 2015 book Doing Good Better, the 2022 book What We Owe the Future, and co-author of the 2020 book Moral Uncertainty.
MacAskill was born William Crouch, in 1987, and grew up in Glasgow. His mother worked for the National Health Service and his father worked in IT; he has two older brothers. MacAskill was educated at Hutchesons' Grammar School, a private school in Glasgow. At the age of 15, after learning about how many people were dying as a result of AIDS, he made the decision to work towards becoming wealthy and giving away half of his money. He also volunteered for a disabled scout group. At the age of 18, during his university studies, MacAskill was exposed to Peter Singer's 1972 essay "Famine, Affluence, and Morality", which later became a guiding principle for his life.
MacAskill earned a BA in philosophy at Jesus College, Cambridge, in 2008; his BPhil at St Edmund Hall, Oxford, in 2010; and his DPhil at St Anne's College, Oxford, in 2014 (spending a year as a visiting student at Princeton University), supervised by John Broome and Krister Bykvist. He then took up a junior research fellowship at Emmanuel College, Cambridge, before taking an associate professorship at Lincoln College, Oxford.