ADHD and anxiety symptoms are highly comorbid in childhood. While worse functional outcomes are typically expected for children with comorbid ADHD and anxiety symptoms, an emerging body of literature has suggested that anxiety symptoms may actually contribute to compensatory effects for executive functioning (EF) skills in children with ADHD symptoms. However, the results of studies investigating this claim have been quite mixed, possibly due to the use of smaller sample sizes and cross-sectional datasets. The current study extends the previous literature by examining the possible compensatory effects of anxiety symptoms in the context of ADHD symptoms on EF abilities (e.g., working memory [WM] and inhibition) both cross-sectionally and longitudinally in a large, well-validated sample.

547 children and adolescents (8-16 years) were included from a population-based sample of twins (CLDRC sample) with enrichment for reading and attention challenges. Participants were retested at a second time point approximately 5 years later. ADHD symptoms (inattention and hyperactivity-impulsivity) were measured by a DSM-based ADHD rating scale, anxiety symptoms were measured by the RCMAS, inhibition was measured by stop-signal reaction time (SSRT), and working memory was measured by Digit Span Backwards (WISC/WAIS-R/III). Covariates included age and sex assigned at birth. Multiple regression models examined cross-sectional and longitudinal associations between ADHD (inattention and H-I) symptoms, anxiety symptoms, and the interaction between ADHD and anxiety symptoms on WM and inhibition abilities.

As expected, higher anxiety, inattention, and H-I symptoms were generally associated with lower inhibition and WM abilities both cross-sectionally and longitudinally. While no significant interactions between ADHD and anxiety symptoms were identified cross-sectionally at Time 1, significant interactions between Time 1 ADHD and anxiety symptoms predicted Time 2 inhibition scores. An inattention x anxiety interaction (p=.002) and a H-I x anxiety (p=.016) interaction significantly predicted Time 2 inhibition. Simple slopes analysis confirmed a compensatory interaction pattern, where ADHD symptoms showed a stronger association with inhibition weaknesses in children without anxiety symptoms compared to those with anxiety symptoms. This suggests that anxiety symptoms may be serving as a compensatory factor for children with ADHD symptoms as compared to their peers without ADHD symptoms.

These findings help clarify a previously mixed literature. Our findings suggest that the compensatory effect of anxiety symptoms on inhibition abilities in children with ADHD symptoms may be a developmental mechanism that takes time to emerge. The fact that the compensatory effect may take time to emerge may explain conflicting results within prior cross-sectional samples. These findings also have implications for research investigating the link between ADHD symptoms and EF abilities, as anxiety symptoms may be an important moderator to consider when attempting to explain why the correlation between ADHD symptoms and EF abilities is often weaker than expected. Finally, clinical implications for this work help to provide empirical evidence to support anecdotal experiences reported by individuals with ADHD and the clinicians who assess them, who often report that anxiety symptoms help them to improve EF performance.