In psychology, developmental stage theories are theories that divide psychological development into distinct stages which are characterized by qualitative differences in behavior. Developmental stage theories are one type of structural stage theory. There are several different views about psychological and physical development and how they proceed throughout the life span. The two main psychological developmental theories include continuous and discontinuous development. In addition to individual differences in development, developmental psychologists generally agree that development occurs in an orderly way and in different areas simultaneously. Stage theories The development of the human mind is complex and a debated subject, and may take place in a continuous or discontinuous fashion. Continuous development, like the height of a child, is measurable and quantitative, while discontinuous development is qualitative, like hair or skin color, where those traits fall only under a few specific phenotypes. Continuous development involves gradual and ongoing changes throughout the life span, with behavior in the earlier stages of development providing the basis of skills and abilities required for the next stages. On the other hand, discontinuous development involves distinct and separate stages, with different kinds of behavior occurring in each stage. Stage theories of development rest on the assumption that development is a discontinuous process involving distinct stages which are characterized by qualitative differences in behavior. They also assume that the structure of the stage is not variable according to each individual; however the time of each stage may vary individually. While some theories focus primarily on the healthy development of children, others propose stages that are characterized by a maturity rarely reached before old age. Ego-psychology The psychosexual stage theory created by Sigmund Freud (b.1856) consists of five distinct stages of Psychosexual development that individuals will pass through for the duration of their lifespan. Four of these stages stretch from birth through puberty and the final stage continues throughout the remainder of life. Erik Erikson (b.1902) developed a psychosocial developmental theory, which was both influenced and built upon by Freud, which includes four childhood and four adult stages of life that capture the essence of personality during each period of development. Each of Erikson's stages include both a positive and negative influences that can go on to be seen later in an individuals life. His theory includes the influence of biological factors on development. Jane Loevinger (b.1918) build on the work of Erikson in her description of stages of