Wood, A. C., Rijsdijk, F. V., Johnson, K. A., Andreou, P., Albrecht, B., Arias-Vasquez, A., Buitelaar, J. K., McLoughlin, G., Rommelse, N. N. J., Sergeant, J. A., Sonuga-Barke, E. J. S., Uebel, H., van der Meere, J. J., Banaschewski, T., Gill, M., Manor, I., Miranda, A., Mulas, F., Oades, R. D., Roeyers, H., Rothenberger, A., Steinhausen, H-C., Faraone, S. V., Asherson, P., & Kuntsi, J.

The relationship between ADHD and key cognitive phenotypes is not mediated by shared familial effects with IQ. (2011) Psychological Medicine, 41, 861-871. DOI:10.1017/S003329171000108X View whole article [Request a copy]

Introduction: Previous twin and sibling studies have identified specific cognitive phenotypes that may mediate the association between genes and the behavioural symptoms of ADHD. -- ADHD is associated with lower IQ scores; the question remains whether lower general cognitive ability could explain the familial & genetic associations observed between ADHD & specific cognitive performance measures.

The aim was to investigate whether the familial association between measures of cognitive performance and a clinical diagnosis of ADHD is mediated through shared influences with IQ.

Methods: Multivariate familial models were run on data from 1265 subjects (6-18y), which comprised of 920 ADHD-sibling pairs and 345 controls. Cognitive assessments included a 4-choice reaction time (RT) task, a Go/no-go task, a choice-delay task and an IQ assessment. The analyses focused on the cognitive variables of mean RT, RT variability, commission errors (CE), omission errors (OE), choice impulsivity (CI).

Results:
1 - Significant familial association was confirmed between cognitive performance and both ADHD (rF:.41-.71) and IQ (rF:-.25--.49).

2 - The association between ADHD & cognitive performance was largely independent (80-87%) of any contribution from etiological factors shared with IQ.

3 - The exception was for CI, where 49% of the overlap could be accounted for by the familial variance underlying IQ.

Discussion: The aetiological factors underlying lower IQ in ADHD appear to be distinct from those between ADHD & RT / error measures.

This suggests that lower IQ does not account for the key cognitive impairments observed in ADHD. [Data on CI warrants further investigation].