Abstract
Background:Fluid intelligence declines with advancing age, starting in early adulthood. Within-subject declines in fluid intelligence are highly correlated with contemporaneous declines in the ability to live and function independently. To support healthy aging, the mechanisms underlying these declines need to be better understood. Methods:In this pre-registered analysis, we applied latent growth curve modelling to investigate the neural determinants of longitudinal changes in fluid intelligence across three time points in 185,317 individuals (N=9,719 two waves, N=870 three waves) from the UK Biobank (age range: 39-73 years). Results:We found a weak but significant effect of cross-sectional age on the mean fluid intelligence score, such that older individuals scored slightly lower. However, the mean longitudinal slope was positive, rather than negative, suggesting improvement across testing occasions. Despite the considerable sample size, the slope variance was non-significant, suggesting no reliable individual differences in change over time. This null-result is likely due to the nature of the cognitive test used. In a subset of individuals, we found that white matter microstructure (N=8839, as indexed by fractional anisotropy) and grey-matter volume (N=9931) in pre-defined regions-of-interest accounted for complementary and unique variance in mean fluid intelligence scores. The strongest effects were such that higher grey matter volume in the frontal pole and greater white matter microstructure in the posterior thalamic radiations were associated with higher fluid intelligence scores. Conclusions:In a large preregistered analysis, we demonstrate a weak but significant negative association between age and fluid intelligence. However, we did not observe plausible longitudinal patterns, instead observing a weak increase across testing occasions, and no significant individual differences in rates of change, likely due to the suboptimal task design. Finally, we find support for our preregistered expectation that white- and grey matter make separate contributions to individual differences in fluid intelligence beyond age.
Highlights
Fluid intelligence refers to the ability to solve novel problems in the absence of task-specific knowledge, and predicts important outcomes including life expectancy, expected income and work performance (Gottfredson & Deary, 2004)
Joint Grey matter and white matter determinants of fluid intelligence we examined whether the grey and white matter provide complementary information about fluid intelligence, in line with our preregistered prediction
Biobank offers a uniquely rich, publicly-available dataset that has revolutionized the scope of large scale shared projects, and already led to numerous insights into the genetic, environmental and neural markers of healthy aging (e.g. Hagenaars et al, 2016; Miller et al, 2016; Muñoz et al, 2016)
Summary
Fluid intelligence refers to the ability to solve novel problems in the absence of task-specific knowledge, and predicts important outcomes including life expectancy, expected income and work performance (Gottfredson & Deary, 2004) Both crosssectional and longitudinal studies have shown that advancing age is associated with a marked decrease in fluid intelligence starting in the third or fourth decade of life (Hartshorne & Germine, 2015; Schaie, 1994). Whereas cross-sectional data typically indicate decline in the third or fourth decade of life (or earlier, see Park et al, 20023) actual decline at the mean level may appear later, at least as judged by the data in Schaie (1994) This is relevant to note as, from that perspective, quite a few participants in the UK biobank study (range 39-73 years) might be expected to be rather stationary in regard to mean-level fluid intelligence over a relatively short test-retest interval. We find support for our preregistered expectation that white- and grey matter make separate contributions to individual differences in fluid intelligence beyond age
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