Adult development theorists believe that the changes that occur during the adult years are predictable and age linked. Their theories explain how change is resolved by the majority of the adult population. Three persons whose research has been influential in the field of adult development during the 1970s are Erik Erikson, Daniel Levinson, and Gail Sheehy. Erikson uses both physiological and societal considerations in charting eight life stages. According to him, each life stage unfolds in sequence; each is triggered by a turning point of increased personal vulnerability and potentiality; and each confronts the individual with central issues demanding resolution. Levinson and his associates have built upon some of the earlier research about life stages and special life stage tasks by studying men aged 18 to 47 and following them for a period of time. They conclude that men develop by periods, and each period engages adults in special tasks that they must master in order to negotiate successfully the tasks of the next period. Gail Sheehy researched the life stories of 115 middle-class Americans aged 18 to 55 to trace inner changes in her subjects, compare the developmental patterns of men and women, and examine the predictable crises for couples. Two other constructs that are critical to an understanding of adult development are role socialization and the midlife crisis or midlife transition. Together, these theories and constructs describe the facets of the adult life cycle. (KC)