Histories of psychology regularly celebrate the foundational role played in the development of early American psychology by Darwin's theory of evolution through natural selection, and in particular the development of functional psychology and behaviorism. In this article it is argued that although Darwin's theory did play an influential role, early American psychology did not generally reflect the hereditarian determinism of his theory of evolution by natural selection. However, early American psychologists did accept one critical implication of Darwin's theory, which is that evolution by natural selection does not ensure the highest development of the human race. This partly explains the social interventive zeal that was a distinctive feature of early American psychology. DOI: 10.1177/0952695107086189