To our knowledge, this is the first study to examine reciprocal longitudinal relations between neuropsychological functioning and dimensionally-assessed ADHD severity among children identified during preschool as being at high or low risk for developing ADHD. In the overall sample, there was an inverse, reciprocal, longitudinal association between ADHD severity and neuropsychological functioning after the age of 4–5 years. Further, there were significant differences between the H/I and TD groups, suggesting that the neurobehavioral trajectory in children at-risk for ADHD may be different from their TD peers. In the H/I group, improved neuropsychological functioning was associated with a subsequent diminution of ADHD severity, providing convincing support for the hypothesis that more optimal neural development is associated with a reduction of symptoms within that age-range; conversely, less optimal neurodevelopment may portend worse symptom presentation during early childhood (Giedd et al., 2010; Halperin et al., 2006; Shaw et al., 2007). Although of lesser magnitude, within that same group, greater ADHD severity at 5–6 years was associated with poorer neuropsychological functioning at 6–7 years, suggesting that entry into formal school may be the time
