Working memory and intra-individual variability in response time mediate fluid intelligence deficits associated with ADHD symptoms

Attention-deficit/hyperactivity disorder (ADHD) is a neuropsychologicalcondition characterised by inattention and hyperactivity. Cognitive deficits are commonly observed in ADHD patients, including impaired working memory, response consistency, and fluid intelligence, which are theorised to be associated with one another. We aimed to determine if decreased fluid intelligence was associated with ADHD, and was mediated by deficits in working memory and intra-individual variability (IIV) in motor responding. The present study tested 142 young adults from the general population on a range of working memory, response time, and fluid intelligence tasks, and an ADHD self-report symptoms questionnaire. Results showed that total and hyperactive ADHD symptoms correlated significantly and negatively with fluid intelligence, but this association was fully mediated by both working memory and IIV in response time. However, inattentive symptoms were not associated with fluid intelligence. These results have important implications for clinicians using speeded psychometric tests as part of their assessment battery.