A theory is proposed to account for some of the age-related differences reported in measures of Type A or fluid cognition. The central hypothesis in the theory is that increased age in adulthood is associated with a decrease in the speed with which many processing operations can be executed and that this reduction in speed leads to impairments in cognitive functioning because of what are termed the limited time mechanism and the simultaneity mechanism. That is, cognitive performance is degraded when processing is slow because relevant operations cannot be successfully executed (limited time) and because the products of early processing may no longer be available when later processing is complete (simultaneity). Several types of evidence, such as the discovery of considerable shared age-related variance across various measures of speed and large attenuation of the age-related influences on cognitive measures after statistical control of measures of speed, are consistent with this theory.The purpose of the current article is to describe, and discuss the evidence relevant to, the processing-speed theory of cognitive aging phenomena. The fundamental assumption in the theory is that a major factor contributing to age-related differences in memory and other aspects of cognitive functioning is a reduction with increased age in the speed with which many cognitive operations can be executed (Salthouse, 1985b). In this article, discussion of evidence relevant to the theory is restricted to the adult portion of the life span, but the basic mechanism may be relevant across the entire life span because similar ideas have been proposed by Kail (e.g., 1986Kail (e.g., , 1991Kail & Park, 1992) regarding the development of cognitive functioning during childhood.Because the success of a theory cannot be evaluated if the goal one hopes to achieve is never clearly specified, I begin by briefly describing the phenomenon that the present theory is intended to explain. Some of the best-documented findings in the literature on aging and cognition are the age-related differences in Type A (Hebb, 1942) or fluid (Cattell, 1943;Horn, 1982;Horn & Cattell, 1963) cognition, which include a wide variety of measures of memory, reasoning, and spatial abilities. The relations between age and cognition have been well documented since the earliest mental testing of adults (e.g., Foster & Taylor, 1920;Jones & Conrad, 1933), and they are readily apparent in the results from the standardization data in psychometric and neuropsychological test batteries (e.g., see Salthouse, 1991c, chap. 2, for a review). Because the samples for the standardization data in these test batteries are typically large and representative, and because the performance measures are Preparation of this article was supported by National Institute on Aging Grant R37 AG06826.1 would like to thank John Dunlosky, Julie Earles, Dan Fisk, Leah Light, Ulman Lindenberger, and David Madden for helpful comments on earlier versions of this article.Correspondence concerning this article sho...

Cross-sectional comparisons have consistently revealed that increased age is associated with lower levels of cognitive performance, even in the range from 18 to 60 years of age. However, the validity of cross-sectional comparisons of cognitive functioning in young and middle-aged adults has been questioned because of the discrepant age trends found in longitudinal and cross-sectional analyses. The results of the current project suggest that a major factor contributing to the discrepancy is the masking of age-related declines in longitudinal comparisons by large positive effects associated with prior test experience. Results from three methods of estimating retest effects in this project, together with results from studies comparing non-human animals raised in constant environments and from studies examining neurobiological variables not susceptible to retest effects, converge on a conclusion that some aspects of age-related cognitive decline begin in healthy educated adults when they are in their 20s and 30s.Keywords cognitive aging; early adulthood; normal aging Although there have been many reports over the last 100 years of age-related differences in cognitive functioning, there is still considerable controversy about the age at which cognitive decline begins. This lack of consensus is unfortunate because the question is important for both practical and theoretical reasons. For example, the age at which cognitive decline begins is relevant to the optimum time to implement interventions designed to prevent or reverse age-related declines. Many interventions currently target adults 60 years of age and older. However, if people start to decline when they are in their 20s and 30s, a large amount of change will likely have already occurred by the time they are in their 60s and 70s. This may affect the likelihood that interventions at that age will be successful because the changes might have accumulated to such an extent that they may be difficult to overcome.The question of when decline begins is also relevant to the theoretical investigation of potential causes of declines in cognitive functioning because declines that begin early are unlikely to be attributable to conditions specific to later life, such as menopause, retirement from paid employment, or certain age-related diseases. The answer to the question of when

Two studies, involving a total of 460 adults between 18 and 87 years of age, were conducted to determine which of several hypothesized processing components was most responsible for age-related declines in working memory functioning. Significant negative correlations between age and measures of working memory (i.e., from-.39 to-.52) were found in both studies, and these relations were substantially attenuated by partialing measures hypothesized to reflect storage capacity, processing efficiency, coordination effectiveness, and simple comparison speed. Because the greatest attenuation of the age relations occurred with measures of simple processing speed, it was suggested that many of the age differences in working memory may be mediated by age-related reductions in the speed of executing elementary operations.

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Critical requirements for the hypothesis that executive functioning is a potential mediator of age-related effects on cognitive functioning are that variables assumed to reflect executive functioning represent a distinct construct and that age-related effects on other types of cognitive functioning are reduced when measures of executive functioning are statistically controlled. These issues were investigated in a study involving 261 adults between 18 and 84 years of age. Although age-related effects on various cognitive abilities were substantially reduced after statistical control of the variance in measures hypothesized to represent executive functioning, there was only weak evidence for the existence of distinct constructs corresponding to executive functioning or to aspects of executive control concerned with inhibition, updating, or time sharing.

A meta-analysis was conducted on 91 studies to derive a correlation matrix for adult age, speed of processing, primary-working memory, episodic memory, reasoning, and spatial ability. Structural equation modeling with a single latent common cognitive factor showed that all cognitive measures shared substantial portions of age-related variance. A mediational model revealed that speed of processing and primary-working memory appear to be important mediators of age-related differences in the other measures. However, not all of the age-related influences were mediated. An examination of quadratic age effects and correlational patterns for subsamples under and over 50 years of age revealed that (a) negative age-cognition relations were significant for the 18- to 50-year-old sample and (b) the age-related decline accelerated significantly over the adult life span for variables assessing speed, reasoning, and episodic memory.

Research concerned with relations between adult age and cognitive functioning is briefly reviewed. The coverage is necessarily selective, and is organized in terms of five major questions. These are what abilities are related to age, how many distinct influences are contributing to the relations between age and cognitive functioning, do the differences between people increase with advancing age, what is responsible for the discrepancies between cross-sectional and longitudinal age comparisons of cognitive functioning, and what methods can be used to identify causes of age-related influences on cognition. Although definitive answers are not yet possible, quite a bit of information relevant to the questions is now available. Moreover, the existing information has implications for the design, analysis, and interpretation of cognitive and neuropsychological research concerned with aging.

ABSTRACT-Adult age differences have been documented on a wide variety of cognitive variables, but the reasons for these differences are still poorly understood. In this article, I describe several findings that will need to be incorporated into eventual explanations of the phenomenon of cognitive aging. Despite common assumptions to the contrary, age-related declines in measures of cognitive functioning (a) are relatively large, (b) begin in early adulthood, (c) are evident in several different types of cognitive abilities, and (d) are not always accompanied by increases in between-person variability.