Are planning, working memory, and inhibition associated with individual differences in preschool ADHD symptoms?

The association between executive function (EF; planning, working memory, and inhibition) and individual differences in symptoms of attention deficit hyperactivity disorder (ADHD) was explored in a sample of preschool children. One hundred sixty children (between the ages of 3 years, 0 months and 5 years, 6 months), selected so as to oversample high ADHD scorers, performed 3 tasks previously shown to measure planning (Tower of London), working memory (Noisy Book) and inhibition ("Puppet Says..."). EF measures were reliable (kappa > .77) and were correlated with IQ (rs > .38) and age (rs > .59). Once IQ and age were controlled, planning and working memory (r = .41) were correlated. Planning and working memory were not correlated with inhibition (rs < .20). There was no association between ADHD and working memory or planning (rs < .12). There was a significant negative association between ADHD and conduct problems and inhibition (r = -.30 and r = -.25, respectively). Only the link with ADHD persisted after the effects of other factors were controlled for in a multiple regression. Specific deficits in inhibitory control rather than general EF deficits are associated with ADHD in the preschool period. This association is linear in nature, supporting the idea that ADHD is better seen as a continuum rather than a discrete category. This association provides evidence for Barkley's (1997) view that ADHD is underpinned by inhibitory deficits in the preschool period.