Adult Age Differences in Covariation of Motivation and Working Memory Performance: Contrasting Between-Person and Within-Person Findings

Developmental theorists have proposed for a long time that the prevailing focus on stable individual differences has obstructed the discovery of short-term covariations between cognitive performance and contextual influences within individuals that may help to uncover mechanisms underlying long-term change. As an initial step to overcome this imbalance, we observed measures of motivation and working memory (WM) in 101 younger and 103 older adults across 100 occasions. Our main goals were to (1) investigate day-to-day relations between motivation and WM, (2) show that these relations differ between groups of younger and older adults, and (3) test whether the within-person and between-person structures linking motivational variables to WM are equivalent (i.e., the ergodicity assumption). The covariation between motivation and WM was generally positive in younger adults. In contrast, older adults showed reduced variability in motivation, increased variability across trials, and small reliability-adjusted correlations between motivation and WM. Within-person structures differed reliably across individuals, defying the ergodicity assumption. We discuss the implications of our findings for developmental theory and design, stressing the need to explore the effects of between-person differences in short-term covariations on long-term developmental change.