As a subset of the science of psychology, developmental psychology is concerned with the study of the various changes that occur over the human life span. While the field originally only studied infants and children, it has expanded in recent years to include the entire range of life, including adolescence, adulthood, and aging. Developmental psychology is an important tool in several disciplines, including education, forensic developmental psychology and child psychopathology.
One of the earliest theorists in developmental psychology was Sigmund Freud, who developed his famous theory of psychosexual development. Freud said there were five stages of development, the oral, anal, phallic, latent and genital stages, which encompass the human lifespan from infancy to the end of adolescence. Erik Erikson added a social dimension to Freud's theory, developing an eight-stage theory of psychosocial development. Erikson's theory extended development into old age.
Similar to Freud, Piaget developed his theory of cognitive development that ended development in adolescence. However, Piaget felt that individuals learn through hands-on experience, pushing cognitive development though four stages: sensorimotor, pre-operational, concrete operational, and formal operational.
One of the major debates in developmental psychology is the relationship between nature and nurture. Is behavior the result of environmental interaction or innate in one's genes? There is yet to be a conclusive answer.