The processing-speed theory of adult age differences in cognition

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 perfor-mance is degraded when processing is slow because relevant operations cannot be successfully exe-cuted (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 con-siderable 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 con-sistent with this theory. The purpose of the current article is to describe, and discuss the evidence relevant to, the processing-speed theory of cogni-tive aging phenomena. The fundamental assumption in the the-ory is that a major factor contributing to age-related differences in memory and other aspects of cognitive functioning is a re-duction with increased age in the speed with which many cog-nitive 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., 1986, 1991; Kail & Park, 1992) regarding the development of cognitive functioning dur-ing 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 differ