Life-Span Development

The description or analysis of simultaneous gains and losses along multiple dimensions across time or age requires sophisticated research methods. Perhaps one of the most important methodological issues derivative from life-span theory has been the distinction between age, cohort, and time of measurement as sources of influence in developmental research. In the study of aging, it is known that observations of age differences in behavior are attributable to both cohort factors (influences associated with time of birth) and to age factors. Further, observations of longitudinal changes across multiple times of measurement are attributable to chronological age change and to the influences of changing socio-historical circumstances. The significance of age, cohort, and socio-historical factors as distinct sources of influence was not really appreciated until development was conceptualized in a broad life-span framework.

Another important methodological theme in life-span developmental theory has to do with the specification and meaning of the age variable. Although age-related change is usually described in terms of chronological age, years since birth provides a crude and unsatisfactory index of many aging phenomena. One theme of the life-span approach is to replace chronological age with index measures that more accurately capture the sources of time-related or age-related change. For example, the effects of aging on behavior are sometimes irreversible. The effects of aging on behavior can also be quantitative and continuous rather than qualitative, and reversible rather than irreversible. Further, markers of elapsed time are insensitive to the meaning of time as a relative and subjective dimension. Indeed, the experience of the passing of 365 days is not likely to be the same for different aged individuals. There are "social clocks" based on culture-related age prescriptions as well as "biological clocks" based on physiological time. Because age and time are not causes of change per se, an aim of aging research is to identify the mechanisms that are primarily responsible for age-related change. Careful description of the social and biological processes that produce aging would enable researchers to replace the index variable, time since birth, with the variables for which it is proxy. For example, there is some evidence to suggest that measures of brain reserve capacity can serve as more accurate measures of the effects of aging than chronological age.