James Mark Baldwin was an early psychologist whose work influenced many of the major psychologists of the twentieth century. His recognition that children's mental growth occurs in stages, similar to their physical growth, was foundational for the field of developmental psychology. As did Baldwin, Unification thought acknowledges the principle of stages of growth, and its application not only to external, physical growth, but also to the development of internal, mental aspects of human life.
Baldwin also proposed a theory of organic selection that can be seen as a unity of the influence of "nature" and "nurture." It is also a theory that intended to unite two evolutionary theories, those of Darwin and Lamarck, each of which was inadequate to explain the complexity of the processes involved. Baldwin's understanding that inheritance involves not only physical attributes and instincts, but also culturally determined behaviors, added a valuable extension to the understanding of the development of human nature.
However, in relation to Unification thought, Baldwin did not include the spiritual aspect of human beings in his work, and did not realize that selfishness is not true human nature but rather the result of the wrong, fallen behavior of the first human ancestors. Thus, although he had remarkable insights into the processes involved in the development of human behavior, he could not identify the problems resulting from the human fall nor their solution.