Attention-Deficit Hyperactivity Disorder: A Category or a Continuum?
Genetic Analysis of a Large-Scale Twin Study
Florence Levy, David A. Hay, Michael McStephen, Catherine Wood, Irwin 
Waldman
Abstract
Objective: 
To investigate heritability and continuum versus categorical approaches 
to attention-deficit hyperactivity disorder (ADHD), using a large-scale 
twin sample. 
Method: 
A cohort of 1,938 families with twins and siblings aged 4 to 12 years, 
recruited from the Australian National Health and Medical Research 
Council Twin Registry, was assessed for ADHD using a DSM-III R-based 
maternal rating scale. Probandwise concordance rates and correlations in 
monozygotic and dizygotic twins and siblings were calculated, and 
heritability was examined using the De Fries and Fulker regression 
technique. 
Results: 
There was a narrow (additive) heritability of 0.75 to 0.91 which was 
robust across familial relationships (twin, sibling, and twin-sibling) 
and across definitions of ADHD as part of a continuum or as a disorder 
with various symptom cutoffs. There was no evidence for nonadditive 
genetic variation or for shared family environmental effects. 
Conclusions: 
These findings suggest that ADHD is best viewed as the extreme of a 
behavior that varies genetically throughout the entire population rather 
than as a disorder with discrete determinants. This has implications for 
the classification of ADHD and for the identification of genes for this 
behavior, as well as implications for diagnosis and treatment.
J. Am. Acad. Child Adolesc. Psychiatry, 1997, 36(6):737-744. 
               The Few. The Proud. The Chosen.
                     markprobe@aol.com
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 * CMPQwk #1.4 * UNREGISTERED EVALUATION COPY
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