Robert Yerkes is a pivotal figure in American psychology and primatology in the first half of the twentieth century. As is well known, Yerkes first studied ape intelligence in 1915, on a visit to the private California laboratory of the psychiatrist Gilbert Hamilton, a former student. Less widely appreciated is how far the work done at the Hamilton lab, in its aims and ambitions as well as its techniques, served as a template for much of Yerkes's research thereafter. This paper uses the Hamilton-Yerkes relationship to re-examine Yerkes's career and, more generally, that of American psychology in the early twentieth century. Three points especially are emphasized: first, the role of Freudian psychoanalysis as a spur to Hamilton's experimental studies of ape intelligence; second, the importance of Hamilton's laboratory, with its semi-wild population of monkeys and ape, as a model for Yerkes's efforts to create a laboratory of his own; and third, the influence on Yerkes of Hamilton's optimism about experimental psychological studies of nonhuman primates as a source of lessons beneficial to a troubled human world.