a1 MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, UK
a2 Department of Epidemiology and Section on Statistical Genetics, University of Alabama at Birmingham, Birmingham, AL, USA
a3 Department of Psychiatry, Trinity Centre for Health Sciences, St James's Hospital, Dublin, Ireland
a4 School of Psychology, Queen's University Belfast, Northern Ireland, UK
a5 Child and Adolescent Psychiatry, University of Göttingen, Germany
a6 Department of Psychiatry, Donders Institute for Brain, Cognition and Behavior, Centre for Neuroscience, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands
a7 Department of Human Genetics, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands
a8 Department of Clinical Neuropsychology, Vrije Universiteit, Amsterdam, The Netherlands
a9 Developmental Brain-Behaviour Laboratory, University of Southampton, UK
a10 Department of Experimental Clinical and Health Psychology, Ghent University, Belgium
a11 Department of Neuropsychology, University of Groningen, The Netherlands
a12 Department of Child and Adolescent Psychiatry and Psychotherapy, Central Institute of Mental Health, University of Heidelberg, Mannheim, Germany
a13 S. Herzog Memorial Hospital, Research Department, Jerusalem, Israel
a14 Department of Developmental and Educational Psychology, University of Valencia, Spain
a15 Department of Neuropediatrics, La Fe University Hospital, Valencia, Spain
a16 Clinic for Child and Adolescent Psychiatry and Psychotherapy, University of Duisburg-Essen, Germany
a17 Department of Child and Adolescent Psychiatry, University of Zurich, Switzerland
a18 Child and Adolescent Clinical Psychology, Institute of Psychology, University of Basel, Switzerland
a19 Child and Adolescent Psychiatry, Psychiatric Hospital Aalborg, University Hospital Aarhus, Aalborg, Denmark
a20 Department of Neuroscience, SUNY Upstate Medical University, Syracuse, NY, USA
a21 Department of Psychiatry, SUNY Upstate Medical University, Syracuse, NY, USA
Background Twin and sibling studies have identified specific cognitive phenotypes that may mediate the association between genes and the clinical symptoms of attention deficit hyperactivity disorder (ADHD). ADHD is also associated with lower IQ scores. We aimed to investigate whether the familial association between measures of cognitive performance and the clinical diagnosis of ADHD is mediated through shared familial influences with IQ.
Method Multivariate familial models were run on data from 1265 individuals aged 6–18 years, comprising 920 participants from ADHD sibling pairs and 345 control participants. Cognitive assessments included a four-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 (MRT), RT variability (RTV), commission errors (CE), omission errors (OE) and choice impulsivity (CI).
Results Significant familial association (rF) was confirmed between cognitive performance and both ADHD (rF=0.41–0.71) and IQ (rF=−0.25 to −0.49). The association between ADHD and cognitive performance was largely independent (80–87%) of any contribution from etiological factors shared with IQ. The exception was for CI, where 49% of the overlap could be accounted for by the familial variance underlying IQ.
Conclusions The aetiological factors underlying lower IQ in ADHD seem to be distinct from those between ADHD and RT/error measures. This suggests that lower IQ does not account for the key cognitive impairments observed in ADHD. The results have implications for molecular genetic studies designed to identify genes involved in ADHD.
(Received January 14 2009)
(Revised April 13 2010)
(Accepted April 21 2010)
(Online publication June 04 2010)