The developmental stage approach refers to the socially or developmentally defined, age-related sequence of stages individuals experience from birth through death. The assumption underlying the stage theories is that each stage represents a qualitatively unique period of development, indicating that the type of development is completely different and not reducible to earlier forms. That is, the developmental process occurs in an invariant sequence. The stage theory assumes that development is cumulative in nature and that development is based on each preceding step. It is expected that development proceeds toward increasingly complex levels of functioning. From a traditional viewpoint, the process of development proceeds in an irreversible sequence. Theoretically, one cannot return to an earlier form of development. The sequence of development is universal in nature. Development proceeds toward predictable end-states. These end-states may be influenced by maturational factors or environmental factors.