Eg. Willcutt et al., A comparison of the cognitive deficits in reading disability and attention-deficit/hyperactivity disorder, J ABN PSYCH, 110(1), 2001, pp. 157-172
This study used a nonreferred sample of twins to contrast the performance o
f individuals with reading disability (RD; n = 93), attention-deficit/hyper
activity disorder (ADHD; n = 52), RD and ADHD (n = 48), and neither RD nor
ADHD (n = 121) on measures of phoneme awareness (PA) and executive function
ing (EF), Exploratory factor analysis of the EF measures yielded underlying
factors of working memory, inhibition, and set shifting. Results revealed
that ADHD was associated with inhibition deficits, whereas RD was associate
d with significant deficits on measures of PA and verbal working memory. Th
e RD + ADHD group was most impaired on virtually all measures, providing ev
idence against the phenocopy hypothesis as an explanation for comorbidity b
etween RD and ADHD.