At the heart of the continuity versus discontinuity debate lies the question of whether development is solely and evenly continuous, or whether it is marked by age‐specific periods. Developmentalists who advocate the continuous model describe development as a relatively smooth process, without sharp or distinct stages, through which an individual must pass. Meanwhile, supporters of the discontinuous model describe development as a series of discrete stages, each of which is characterized by at least one task that an individual must accomplish before progressing to the next stage. For example, Freud, in his stage model of psychosexual development, theorized that children systematically move through oral, anal, phallic, and latency stages before reaching mature adult sexuality in the genital stage. Theories of human development, according to Freud and Erikson, appear in Table . Table shows Piaget's stages of cognitive development, and Table outlines Levinson's stages of passage from age 17 to 65 and over.
Proponents of stage theories of development also suggest that individuals go through critical periods, which are times of increased and favored sensitivity to particular aspects of development. For example, early childhood (the first 5 years) is a critical period for language acquisition. Thus, most adults find it difficult or impossible to master a second language during their adult years while young children raised in bilingual homes normally learn second languages easily during childhood.
Experts from a variety of disciplines continue to argue over the roles that biology and the environment ultimately play in development. This centuries‐old nature‐versus‐nurture debate concerns the relative degree to which heredity and learning affect functioning. Both genetic traits and environmental circumstances are likely to be involved in an individual's development, although the amount each express depends on the individual and his or her circumstances. For example, some identical twins who are separated at birth develop similar personality, cognitive, and social characteristics, while other twins who are separated at birth do not. Likewise, many non‐twin siblings raised in the same household develop similar characteristics, although this similar development of characteristics is not always the case with non‐twin siblings. This interactional nature‐versus‐nurture or biology‐versus‐environment approach to the study of human psychological development exemplifies the multifaceted makeup of the biopsychosocial perspective.