Psychological development. Psychological development, the development of human beingsâ cognitive, emotional, intellectual, and social capabilities and functioning over the course of the life span, from infancy through old age. It is the subject matter of the discipline known as developmental psychology. Child psychology was the traditional focus of research, but since the mid-20th century much has been learned about infancy and adulthood as well. A brief treatment of psychological development follows. For full treatment, see human behaviour.