Children’s cognitive performance varies from moment-to-moment in a way that is shaped by, and in turns shapes, their development. What drives this variation remains poorly understood. One field where within-person variability in cognitive performance has seen unprecedented interest, may hold some answers. For a large proportion of children with Attention/Deficit Hyperactivity disorder, high within-person variability in reaction times is considered a key cognitive characteristic. This finding is accompanied by a rich theoretical literature that offers empirically testable predictions. However, reliance on suboptimal methods and datasets has obstructed insight from empirical tests. We identify three compounding sources of heterogeneity that reside in: statistical estimation, cognitive tasks, and psychopathology measurement. We address this heterogeneity to revisit pertinent theoretical questions. First, we isolate a theoretically motivated estimate of reaction-time variability from often confounded cognitive characteristics, using dynamic structural equation modeling. We use this estimate of reaction-time variability to test the specificity of its association with Attention/Deficit Hyperactivity disorder, over-and-above other developmental problems in a population-based cohort of 1032 children aged 5.5 to 13.5. We find that reaction-time variability is specifically associated with symptom-severity in the inattention domain. Second, we test four hypotheses about mechanisms driving reaction-time variability, and their relation with individual differences in inattention and hyperactivity/impulsivity severity. We use a unique task design in combination with latent difference score models to isolate the effect of distinct mechanisms on reaction-time variability. Our findings support changes in attentiveness as a mechanism driving reaction-time variability. We show how a close dialogue between neuroscience, clinical psychology, and psychometrics can accelerate our understanding of within-person variability in cognitive performance.