David A. Hay1, M. McStephen, 1,2, I. Waldman,3, F. Levy ,4
Compared with the DSM-III-R unitary view of Attention Deficit Hyperactivity Disorder (ADHD), DSM-IV made radical change in defining three distinct subtypes of ADHD, namely the Predominantly Inattentive(PI), the Hyperactive/Impulsive (HI) and the Combined (C) subtypes. Given the exceptionally high genetic component to ADHD (F.Levy, D.A. Hay, M. McLaughlin, C. Wood and I. Waldman, 1997, J.Amer. Acad. Child Adol. Psych. 36737-744), MZ and DZ concordances should provide some validation that these three subtypes can be reliably distinguished. The Australian Twin ADHD Project (ATAP) is a longitudinal study of some 2000 families in the Australian NHMRC Twin Registry and the data here come from the 1167 same-sex twin pairs in the 1994-5 second wave when they were aged 8-16. Parents completed our Australian Disruptive Behaviours Scale, which is a conservative measure of ADHD symtomatology and diagnosis (Levy et al. 1997). Proband-wise concordances and cross-concordances between the subtypes indicated that three subtypes were essentially distinct genetically with the MZ cross-concordances being 0.10 or less. DSM-IV requires 6/9 symptoms and the distributions of subtype symptoms in the cotwins of the probands provides additional validation for such cutoff scores. The age distributions for the subtypes were very similar, providing no support for the view (R. Barkley, 1997, Psych. Bull. 121, 65-94) that the HI subtype is a developmental precursor of the C subtype. The phenotypical and genetic relationship of the subtypes to speech and reading problems represented further validation of these distinctions. Rates of intervention for these problems were very low in the HI subtype, but 20% in the PI and 50% in the C subtype. The existence of a genetically discrete subtype combining both Inattention and Hyperactivity/Impulsivity is a challenge for conventional multivariate genetics that seeks to model the relationship between these groups of symptoms.
Address: School of Psychology, Curtin University of Technology,PO Box U 1987, Perth, WA 6845, Australia Phone 61 8 9266 7025 Fax 61 8 9266 2464, email d.hay@psychology.curtin .edu.au
1School of Psychology, Curtin University of Technology, Perth, WA 6845,Australia 2Mental Health Research Institute of Victoria, Parkville, VIC 3152, Australia 3Department of Psychology, Emory University, Atlanta,GA, 30322, USA 4Avoca Clinic, Prince of Wales Hospital, Randwick, NSW, 2031, Australia 5Funded by the National Health and Medical Research Council of Australia