Copyright © 1999 Massachusetts Institute of Technology
Any discussion of the relations between aging and cognition must acknowledge a distinction between two types of cognition that are sometimes referred to as fluid and crystallized cognitive abilities (Cattell 1972) or INTELLIGENCE. Fluid abilities include various measures of reasoning (including both CAUSAL REASONING and DEDUCTIVE REASONING), MEMORY, and spatial performance, and can be characterized as reflecting the efficiency of processing at the time of assessment. In contrast, crystallized abilities are evaluated with measures of word meanings, general information, and other forms of knowledge, and tend to reflect the accumulated products of processing carried out in the past.
The distinction between these two types of abilities is important because the relations of age are quite different for the two forms of cognition. That is, performance on crystallized measures tends to remain stable, or possibly even increase slightly, across most of the adult years, whereas increased age is associated with decreases in many measures of fluid cognition. In large cross-sectional studies age-related declines in fluid abilities are often noticeable as early as the decade of the thirties, and the magnitude of the difference across a range from twenty to seventy years of age is frequently one to two standard deviation units. Although the average trends can be quite large, it is also important to point out that individual differences are substantial because chronological age by itself seldom accounts for more than 20 to 30 percent of the total variance in the scores.
The vast majority of the research in the area of aging and cognition has focused on fluid abilities. There appear to be two primary reasons for this emphasis. First, many researchers probably believe that explanations are clearly needed to account for the differences that have been reported (as in fluid abilities), but that a lack of a difference (as in crystallized abilities) does not necessarily require an explanation. And second, because fluid abilities are assumed to reflect the individual's current status, they are often considered to be of greater clinical and practical significance than crystallized abilities that are assumed to represent the highest level the individual achieved at an earlier stage in his or her life.
Both distal and proximal interpretations of the age-related decline in fluid cognitive abilities have been proposed. Distal interpretations focus on factors from earlier periods in the individual's life that may have contributed to his or her level of performance at the current time. Examples are speculations that the age-related declines are attributable to historical changes in the quantity or quality of education, or to various unspecified cultural characteristics that affect cognitive performance. In fact, comparisons of the scores of soldiers in World War II with the norms from World War I (Tuddenham 1948), and a variety of time-lag comparisons reported by Flynn (1987), suggest that the average level of cognitive ability has been improving across successive generations. However, the factors responsible for these improvements have not yet been identified (see Neisser 1997), and questions still remain about the implications of the positive time-lag effects for the interpretation of cross-sectional age differences in cognitive functioning (see Salthouse 1991). Hypotheses based on differential patterns of activity and the phenomenon of disuse can also be classified as distal because they postulate that an individual's current level of performance is at least partially affected by the nature and amount of activities in which he or she has engaged over a period of years. Although experientially based interpretations are very popular among the general public (as exemplified in the cliché "Use it or lose it") and among many researchers, there is still little convincing evidence for this interpretation. In particular, it has been surprisingly difficult to find evidence of interactions of age and quality or quantity of experience on measures of fluid cognitive abilities that would be consistent with the view that age-related declines are minimized or eliminated among individuals with extensive amounts of relevant experience.
Proximal interpretations of age-related differences in cognitive functioning emphasize characteristics of processing at the time of assessment that are associated with the observed levels of cognitive performance. Among the proximal factors that have been investigated in recent years are differences in the choice or effectiveness of particular strategies, differences in the efficiency of specific processing components, and alterations in the quantity of some type of processing resource (such as WORKING MEMORY, ATTENTION, or processing speed), presumed to be required for many different types of cognitive tasks. Hypotheses based on speculations about the neuroanatomical substrates of cognitive functioning (such as dopamine deficiencies or frontal lobe impairments) might also be classified as proximal because they have primarily focused on linking age-related changes in biological and cognitive characteristics, and not in speculating about the origins of either of those differences. However, not all neurobiological mechanisms are necessarily proximal because some may operate to affect the susceptibility of structures or processes to changes that occur at some time in the future.
A fundamental issue relevant to almost all proximal interpretations concerns the number of distinct influences that are contributing to age-related differences in cognitive functioning. Moderate to large age-related differences have been reported on a wide variety of cognitive variables, and recent research (e.g., Salthouse 1996) indicates that only a relatively small proportion of the age-related effects on a given variable are independent of the age-related effects on other cognitive variables. Findings such as these raise the possibility that a fairly small number of independent causal factors may be responsible for the age-related differences observed in many variables reflecting fluid aspects of cognition. However, there is still little consensus regarding the identity of those factors or the mechanisms by which they exert their influence.
Cattell, R. B. (1972). Abilities: Their Structure, Growth, and Action. Boston: Houghton Mifflin.
Flynn, J. R. (1987). Massive IQ gains in 14 nations: what IQ tests really measure. Psychological Bulletin 101:171-191.
Neisser, U. (1997). Rising scores on intelligence tests. American Scientist 85:440-447.
Salthouse, T. A. (1991). Theoretical Perspectives on Cognitive Aging. Mahwah, NJ: Erlbaum.
Salthouse, T. A. (1996). Constraints on theories of cognitive aging. Psychonomic Bulletin and Review 3:287-299.
Tuddenham, R. D. (1948). Soldier intelligence in World Wars I and II. American Psychologist 3:54-56.
Blanchard-Fields, F., and T. M. Hess. (1996). Perspectives on Cognitive Change in Adulthood and Aging. New York: McGraw-Hill.
Craik, F. I. M., and T. A. Salthouse. (1992). Handbook of Aging and Cognition. Mahwah, NJ: Erlbaum.
Copyright © 1999 Massachusetts Institute of Technology