In July 2024, President Joe Biden withdrew from his campaign for reelection to the US Presidency largely due to concerns over his apparent decline in cognitive abilities. But what do we actually know about how our cognitive abilities decline as we age? Breit and colleagues decided to examine the stability of eleven major cognitive abilities over the lifespan from preschool age to late adulthood (80 and older). They analyzed the results of 205 studies testing over 87 thousand individuals at various points in their lives, using a test-retest procedure. As we might expect, they found that cognitive abilities grow rapidly during the first 20 years of life and then become remarkably stable for the remainder of the lifespan. Nevertheless, significant individual differences in cognitive stability appeared in later adulthood with some older individuals (55 to 90 years of age) showing greater declines in cognitive ability than others in the same age group, with the biggest individual differences in cognitive stability appearing around age 70.
Fluid versus Crystallized Cognitive Abilities
One particularly important finding was that reasoning abilities that require cognitive effort (fluid reasoning) were more likely than knowledge-based abilities (crystallized intelligence) to decline as people aged. The authors explained this somewhat counter-intuitive result as follows:
This theory proposes that during cognitive development, fluid (or effortful processing–based) abilities are invested in the acquisition of crystallized (or knowledge-based) abilities. As the result of years of cumulative investment, these crystallized abilities are acquired and automated, such that they are better maintained even as currently available processing power wanes with aging…or varies from day to day. [p.424]
President Joe Biden, prior to his decision to end his reelection campaign, similarly argued that while he was indeed aging, he had accumulated vast wisdom and political know-how; that is, crystallized intelligence. He was probably right. Voters’ concern of course was that a President also needs to be able to reason through new problems that inevitably arise from time-to-time, if not daily.
Abstract
Cognitive abilities, including general intelligence and domain-specific abilities such as fluid reasoning, comprehension knowledge, working memory capacity, and processing speed, are regarded as some of the most stable psychological traits, yet there exist no large-scale systematic efforts to document the specific patterns by which their rank-order stability changes over age and time interval, or how their stability differs across abilities, tests, and populations. Determining the conditions under which cognitive abilities exhibit high or low degrees of stability is critical not just to theory development but to applied contexts in which cognitive assessments guide decisions regarding treatment and intervention decisions with lasting consequences for individuals. In order to supplement this important area of research, we present a meta-analysis of longitudinal studies investigating the stability of cognitive abilities. The meta-analysis relied on data from 205 longitudinal studies that involved a total of 87,408 participants, resulting in 1,288 test–retest correlation coefficients among manifest variables. For an age of 20 years and a test–retest interval of 5 years, we found a mean rank-order stability of ρ = .76. The effect of mean sample age on stability was best described by a negative exponential function, with low stability in preschool children, rapid increases in stability in childhood, and consistently high stability from late adolescence to late adulthood. This same functional form continued to best describe age trends in stability after adjusting for test reliability. Stability declined with increasing test–retest interval. This decrease flattened out from an interval of approximately 5 years onward. According to the age and interval moderation models, minimum stability sufficient for individual-level diagnostic decisions (rtt = .80) can only be expected over the age of 7 and for short time intervals in children. In adults, stability levels meeting this criterion are obtained for over 5 years.
Citation
Breit, M., Scherrer, V., Tucker-Drob, E. M., & Preckel, F. (2024). The stability of cognitive abilities: A meta-analytic review of longitudinal studies. Psychological Bulletin, 150(4), 399–439.
Dr Martin Fracker 