Attention-Deficit Hyperactivity Disorder Dimensions: A Twin Study of 
Inattention and Impulsivity-Hyperactivity
Dianne K. Sherman, William G. Iacono, Matthew K. McGue
Abstract
Objective: 
This study used a model-fitting strategy to estimate genetic and 
environmental contributions to the core behavioral dimensions associated 
with attention-deficit hyperactivity disorder (ADHD) in 576 twin boys, 
aged 11 and 12 years. 
Method: 
Teacher ratings and maternal structured interview reports composed of 
behavioral items including DSM-III and DSM-III R criteria for ADHD were 
obtained for 194 pairs of monozygotic and 94 pairs of dizygotic twins. 
Factor analysis of these measures yielded two ADHD-related dimensions, 
inattention and impulsivity-hyperactivity. Scales representing these 
dimensions were used in the genetic analyses. 
Results: 
Univariate analyses supported a substantial contribution of genetic 
factors in the expression of inattention and impulsivity-hyperactivity 
and smaller contributions of shared and nonshared environmental factors. 
Results varied according to informant source, with mothers' reports 
suggestive of rater bias effects. Bivariate analyses indicated that the 
correlation between these two ADHD dimensions was also genetically 
mediated. 
Conclusions: 
Genetic factors are etiologically important in the expression of the 
separate dimensions of ADHD and in the covariation between them. 
However, it is important to obtain reports from more than one informant 
because rater bias effects may be operative, particularly in maternal 
reports.
J. Am. Acad. Child Adolesc. Psychiatry, 1997, 36(6):745-753. 
               The Few. The Proud. The Chosen.
                     markprobe@aol.com
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 * CMPQwk #1.4 * UNREGISTERED EVALUATION COPY
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