All the forces we have discussed combine to cre­ate people’s developmental experiences. One way to consider these combinations is to consider the degree to which they are common or unique to people of specific ages. An important concept in this approach is cohort. A cohort is a group of people born at the same point or specific time span in historical time. So everyone born in 1985 would be the 1985 cohort; similarly, those born between 1946 and 1964 represent the baby-boom cohort. Based on this approach, Baltes (1987; Baltes et al., 2006) iden­tifies three sets of influences that interact to produce developmental change over the life span: normative age-graded influences, normative history-graded influences, and nonnormative influences.

Normative age-graded influences are experiences caused by biological, psychological, and socioculturalforces that occur to most people of a particular age. Some of these, such as puberty, menarche, and menopause, are biologi­cal. These normative biological events usually indicate a major change in a person’s life; for example, menopause is an indicator that a woman can no longer bear children without medical intervention. Normative psychological events include focusing on certain concerns at different points in adulthood, such as a middle-aged person’s concern with socializing the younger generation. Other normative age-graded influences involve sociocultural forces, such as the time when first marriage occurs and the age at which someone retires. Normative age-graded influences typically correspond to major time-marker events, which are often ritualized. For example, many younger adults formally celebrate turning 21 as the official transition to adulthood, getting married typically is surrounded with much celebration, and retirement often begins with a party celebrating the end of employ­ment. These events provide the most convenient way to judge where we are on our social clock.

Normative history-graded influences are events that most people in a specific culture experience at the

same time. These events may be biological (such as epidemics), psychological (such as particular stereo­types), or sociocultural (such as changing attitudes toward sexuality). Normative history-graded influ­ences often give a generation its unique identity, such as the baby-boom generation (people born roughly between 1946 and 1964), generation X (people born roughly between 1965 and 1975), and the millennial generation (sometimes called the Echo Boomers or generation Y, born between 1979 and 1994). These influences can have a profound effect. For example, the attacks on the World Trade Center on September 11, 2001, fundamentally changed attitudes about safety and security that had been held for decades.

Nonnormative influences are random or rare events that may be important for a specific individual but are not experienced by most people. These may be favorable events, such as winning the lottery or an election, or unfavorable ones, such as an accident or layoff. The unpredictability of these events makes them unique. Such events can turn one’s life upside down overnight.

Life-cycle forces are especially key in understand­ing the importance of normative age-graded, norma­tive history-graded, and nonnormative influences. For example, history-graded influences may produce generational differences and conflict; parents’ experi­ences as young adults in the 1960s and 1970s (before AIDS, instant messaging, and global terrorism) may have little to do with the complex issues faced by today’s young adults. In turn, these interactions have important implications for understanding differences that appear to be age related. That is, differences may be explained in terms of different life experiences (normative history-graded influences) rather than as an integral part of aging itself (normative age-graded influences). We will return to this issue when we dis­cuss age, cohort, and time-of-measurement effects in research on adult development and aging.