Developmental psychologists are interested in time- and age-related changes in cognitive and intellectual functioning, personality, and social relationships from birth to death. Theory and research deal with three core phenomena: general principles of developmental change, individual differences in development, and intervention possibilities. Two research designs are used to examine these phenomena: cross-sectional and longitudinal studies. Consider, for example, the study of intelligence across the life span. In order to determine general principles of age-related change in intelligence, a cross-sectional design that compares the performance of various age groups (e.g., children, adolescents, young and older adults) on the same test could be used. If children and older adults show a lower test performance compared with the other age groups, one could infer that intelligence increases with age in early life and declines in old age. However, other factors, such as historical changes in education, could also explain the low performance of older adults. Age group differences observed in cross-sectional studies are confounded with cohort differences in life experience and life contexts. Longitudinal studies in which the same individuals are repeatedly measured over time on the same test provide the best assessment of how performance changes with age.
Both cross-sectional and longitudinal research designs are used to investigate individual differences in development. Developmental psychologists ask, for example, whether family backgrounds are linked to individual differences in intellectual development, why some children show delayed or slower growth in intellectual abilities compared with their age peers, and why some adults remain cognitively fit into old age and others show cognitive decline. In addition, developmental researchers are interested in the extent to which cognitive performance (e.g., memory, reasoning, knowledge) can be enhanced at different points in the life course. Carefully designed intervention and training studies are important tools in this respect. Learning about the modifiability (reserve capacity or plasticity) of the cognitive system at different ages helps in better understanding the processes underlying intellectual and cognitive functioning across the life span.
There is general consensus about the importance of investigating three systems of influence on development: age-graded, history-graded, and nonnormative. Each of these systems involves biological and environmental components that contribute to similarities in development as well as to subgroup variations. Age-graded influences include biological and physical changes (e.g., puberty, menopause) as well as exposure to age-related social factors (e.g., schooling, family life cycle, retirement). History-graded influences imply changes in societal structure and function (e.g., economic depression, medical and technical modernization, periods of war or political oppression). Longitudinal research on the developmental trajectories of men and women who were either young children or adolescents during the 1930s economic depression in North America exemplifies this approach (e.g., Elder). Nonnormative influences are conditions that are not associated with chronological age or historical time, but affect an individual's development in important ways (e.g., a lottery win, loss of a leg in an accident).
There is no unified theoretical framework of developmental psychology. Major metatheoretical positions emphasize cognitive structural, biogenetic, psychoanalytical, action-theoretical, social learning, transactional, contextualist, dialectical, and dynamic systems perspectives (for reviews see Bornstein and Lamb; Cairns).
There has been a long-standing debate in the psychological literature about what aspects of change define development and whether or not development occurs across the life span or only in early life (for reviews see Bengtson and Schaie; Cairns; Valsiner). Traditionally, developmental psychology focused primarily on the description and explanation of positive changes (e.g., increased adaptive capacity or growth) in the structure and function of mind and behavior. Change, within this tradition, is considered to reflect development if one or more of the following criteria are met: (1) it is directed toward a state of maturity; (2) it is quantitative and qualitative (stagelike) in nature; (3) it is relatively robust or irreversible; and/or (4) it moves toward greater complexity and differentiation. Using these criteria to define development encourages theoretical precision but also restricts the concept primarily to growth in early life. Is change observed during adulthood and old age associated with development or with processes of aging?
The life-span approach outlined by Paul B. Baltes (1997; Baltes, et al.) proposes that development is not completed at young adulthood (maturity) but extends across the entire life course. Each age period (e.g., infancy, adolescence, adulthood, old age) has its own developmental tasks. When viewed together, however, these age-specific phenomena contribute to continuous (cumulative) and discontinuous (innovative) change throughout life. The great regularity of development observed in infancy and childhood may be attributed to the fact that the biological and cultural influences that shape childhood are more programmed (genetically and societally) than is true for late adulthood. In old age, the conjoint dynamics of biological and cultural influences are less well-orchestrated, in part because the culture of old age is still evolving.
The life-span approach of Baltes and colleagues alerts researchers to the fact that development can be multidirectional in that it involves trajectories of positive growth, stability, and negative change (loss) across the life span. A classic example of this concept is longitudinal research on the trajectories of fluid versus crystallized intelligence during adulthood and into old age (e.g., Schaie). Dimensions of fluid intelligence (e.g., spatial ability, reasoning, perceptual speed) generally show decline beginning in middle age, whereas aspects of crystallized intelligence (e.g., knowledge) remain relatively stable up to at least age eighty.
Expanding the concept of development from a growth model to a multidirectional model led to the insight that development is likely always a combination of gains and losses. A gain in one direction, for example, may exclude alternative pathways of development. The search for gains and losses across the life span has led to much recent research on the plasticity of mind and behavior; the fundamental role of processes of selection, optimization, and compensation in development; and profiles of successful aging.
See also Developmental Tasks; Life Course; Life-Span Development; Selective Optimization with Compensation Theory.
Baltes, P. B. "On the Incomplete Architecture of Human Ontogeny: Selection, Optimization, and Compensation as Foundation of Developmental Theory." American Psychologist 52 (1997): 366–380.
Baltes, P. B.; Lindenberger, U.; and Staudinger, U. M. "Life-Span Theory in Developmental Psychology." In Handbook of Child Psychology. Vol. 1, Theoretical Models of Human Development, 5th ed. Edited by R. M. Lerner. New York: Wiley, 1998. Pages 1029–1143.
Bengtson, V. L., and Schaie, K. W., eds. Handbook of Theories of Aging. New York: Springer, 1999.
Bornstein, M. H., and Lamb, M. E., eds. Developmental Psychology: An Advanced Textbook, 4th. ed. Mahwah, N.J.: Erlbaum, 1999.
Cairns, R. B. "The Making of Developmental Psychology." In Handbook of Child Psychology. Vol. 1, Theoretical Models of Human Development, 5th ed. Edited by R. M. Lerner. New York: Wiley, 1998. Pages 25–106.
Elder, G. H. "The Life Course and Human Development." In Handbook of Child Psychology. Vol. 1, Theoretical Models of Human Development, 5th ed. Edited by R. M. Lerner. New York: Wiley, 1998. Pages 939–991.
Schaie, K. W. Intellectual Development in Adulthood: The Seattle Longitudinal Study. New York: Cambridge University Press, 1996.
Valsiner, J. "The Development of the Concept of Development: Historical and Epistemological Perspectives." In Handbook of Child Psychology. Vol. 1, Theoretical Models of Human Development, 5th ed. Edited by R. M. Lerner. New York: Wiley, 1998. Pages 198–232.
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