William McDougall (22 June 1871 in Chadderton, Lancashire, England - 28 November 1938 Durham, U.S.) was an early twentieth century psychologist who spent the first part of his career in the United Kingdom and the latter part in the United States. He wrote a number of highly influential textbooks, and was particularly important in the development of the theory of instinct and of social psychology in the English-speaking world. He was an opponent of behaviorism and stands somewhat outside the mainstream of the development of Anglo-American psychological thought in the first half of the twentieth century; but his work was very well known and respected among lay people.

McDougall was educated at Owens College, Manchester and St John's College, Cambridge. He also studied medicine and physiology in London and Göttingen. After teaching at University College London and Oxford, he was recruited by William James to Harvard University, where he served as a professor of psychology from 1920 to 1927. He then moved to Duke University, where he established the Parapsychology Laboratory under J. B. Rhine, and where he remained until his death. He was a Fellow of the Royal Society. Among his students was Cyril Burt.

McDougall's interests and sympathies were broad. He was interested in eugenics, but departed from Darwinian orthodoxy in maintaining the possibility of the inheritance of acquired characteristics, as suggested by Jean-Baptiste Lamarck; he carried out many experiments designed to demonstrate this process. Opposing behaviourism, he argued that behaviour was generally goal-oriented and purposive, an approach he called hormic psychology; however, in the theory of motivation, he defended the idea that individuals are motivated by a significant number of inherited instincts, whose action they may not consciously understand, so they might not always understand their own goals. His ideas on instinct strongly influenced Konrad Lorenz, though Lorenz did not always acknowledge this. McDougall underwent psychoanalysis with C. G. Jung, and was also prepared to study parapsychology; in 1920 he served as president of the Society for Psychical Research, and in the subsequent year of its US counterpart, the American Society for Psychical Research.

Because of his interest in eugenics and his unorthodox stance on evolution, McDougall has been adopted as an iconic figure by proponents of a strong influence of inherited traits on behaviour, some of whom are regarded by most mainstream psychologists as scientific racists. While McDougall was certainly an unorthodox figure and always willing to take a minority view, there is no reason to suppose that in the light of modern psychological knowledge and political developments, he would have supported the position taken by these groups. Though he wrote: "...; the few distinguished Negroes, so called, of America - such as Douglass, Booker Washington, Du Bois - have been, I believe, in all cases mulattoes or had some proportion of white blood. We may fairly ascribe the incapacity of the Negro race to form a nation to the lack of men endowed with the qualities of great leaders, even more than to the lower level of average capacity" (McDougall, William., The Group Mind, p.187, Arno Press, 1973; Copyright, 1920 by G.P. Putnam's Sons).

McDougall married at the age of 29 ("against my considered principles", he reports in his autobiographical essay, "for I held that a man whose chosen business in life was to develop to the utmost his intellectual powers should not marry before forty, if at all"). He had five children.