Background
Intelligence quotient (IQ) has traditionally been considered a protective factor against attention-deficit/hyperactivity disorder (ADHD). However, recent findings suggest a more complex relationship. This study aimed to elucidate the nuanced associations between IQ and ADHD. We leveraged a high-quality, multi-dimensional dataset encompassing behavioural, cognitive, and neuroimaging measures from a large ADHD cohort to investigate these relationships comprehensively.
Methods
We conducted a study with 5,138 children, aged 6–16 years, who have been professionally diagnosed with ADHD and have a wide range of IQ levels. General Linear Models were used to fit the relationship between IQ [Full-Scale IQ (FSIQ), Verbal IQ (VIQ), and Performance IQ (PIQ)] and ADHD core symptoms [total scores (TO), inattention (IA), hyperactivity/impulsivity (HI)] while considering sex and age as covariates. Furthermore, we examined if the complex associations between IQ and ADHD symptoms could be validated in executive functions and brain structure associated with ADHD. Finally, Latent class analysis was used to identify IQ and symptomatic characteristics in profiles of functional impairment of ADHD.
Results
Consistent with previous research, our study demonstrated a linear negative correlation between FSIQ and ADHD core symptoms. However, PIQ exhibited an inverted U-shaped relationship with HI scores, peaking at a PIQ of 103. These findings were partially corroborated in the relationships between IQ and executive functions, as well as IQ and brain surface area. Latent profile analysis identified three distinct ADHD profiles: Mild Impairment, Severe Academic Deficit, and Social-Academic Deficit. Notably, while higher IQ was associated with less impairment in academic performance, it did not consistently predict social functional impairment. Paradoxically, the group with the least overall impairment exhibited the highest levels of hyperactivity/impulsivity symptoms.
Conclusion
This study elucidates the intricate relationship between IQ and ADHD, revealing that elevated Performance IQ is associated with increased hyperactivity/impulsivity symptoms, while higher symptom levels may not always indicate worse outcomes.