John Broadus Watson (1878-1958) was an American Psychologist who established the psychological school of behaviorism. He is famous for boasting, facetiously, that he could take any 20 human infants, and by applying behavioural techniques, could create whatever kind of person ("beggar, butcherman, thief") he desired. Naturallly, he admitted that this claim was far beyond his means--noting, merely, that earlier psychologists had made such claims for decades.
With his behaviorism, Watson put the emphasis on external behaviour of people and their reactions on given situations, rather than the internal, mental state of those people. In his opinion, the analysis of behaviours and reactions was the only objective method to get insight in the human actions.
Watson was asked to leave the faculty position he held at Johns Hopkins University because he was having an affair with a student, and subsequently began working for J. Walter Thompson, an advertising agency.
Watson has become immortalized in introductory psychology textbooks for his attempts to condition fear of a white rat into "Little Albert", a 9 month old boy. As the story of Little Albert has made the rounds, inaccuracies and inconsistencies have crept in, some of them even due to Watson himself; see Harris for an analysis.