Levinson proposed that adult development goes through a series of predictable stages, known as seasons or eras. These include: early adulthood (age twenty-two to forty), middle adulthood (age forty to sixty) and late adulthood (age sixty and up). He also proposed the notion of cross-era transitions, in which the adult negotiates the psychological challenges of transitioning from one stage to another. Cross-era transitions last about five to seven years. Levinson suggests that the first stage, early adulthood, is characterized by the creation of the initial life structure. This is a period of great excitement and satisfaction, but also of notable uncertainty and anxiety. To Levinson, adult development involves a series of fluctuations between the creation and rearrangement of life structures. A life structure refers to the entire design of a person’s life, including psychological traits, social relationships, and work life. A life structure is most satisfying when personal needs can be harmonized with external societal demands.