ADHD IS STRONGLY LINKED TO GENES, STUDY FINDS
A study of twins suggests that Attention Deficit/Hyperactivity Disorder 
(ADHD) has a very strong tendency to run in families -- not only as a 
clinical disorder but also as a personality trait. The new research 
appears in the June issue of the Journal of the American Academy of 
Child and Adolescent Psychiatry. 
Heritability for ADHD -- meaning that if one twin has it, the other will 
too -- ranges from 75 to 91 percent, depending on the definition of ADHD 
used. The results also suggest that ADHD is less a distinct disorder 
that is either present or absent than the extreme of a trait that varies 
genetically throughout the population.
The study analyzed data on 1,938 Australian families with twins and 
other children aged four to twelve years old. When at least one twin had 
ADHD, there was a 91 percent chance that the other identical twin would 
have it as well. In contrast, the growing up in the same environment had 
a relatively small effect on the heritability of ADHD, estimated at only 
13 percent. The research was led by Dr. Florence Levy of the University 
of New South Wales, Australia.
Heritability for ADHD was much higher than estimated for other types of 
behavior disorders that run in families. This was so for not only 
children who actually had ADHD, but also for those who had a tendency or 
trait toward ADHD but didn't have enough symptoms for a clinical 
diagnosis.
That finding suggests that children with ADHD are not so much a group 
with a specific disorder but the extreme end of a group with a trait 
toward ADHD. Heritability for the trait toward ADHD was estimated at 75 
percent.
Correlations were somewhat higher for twins than for other siblings, 
suggesting that part of the heritability of ADHD may be specific to 
twins. There was no difference in heritability between identical versus 
nonidentical twins, however.
The findings have important implications for the diagnosis and treatment 
of ADHD, Standard diagnostic approaches rely heavily on the number of 
ADHD symptoms present --for example, children with eight symptoms would 
be diagnosed as having ADHD, whereas using DSM-III-R those with seven 
might not. ADHD may be less a distinct disorder than a deviation from an 
acceptable norm, the findings suggest.
The results may also influence treatment decisions. More flexible drug 
treatment schedules may be called for, depending on the person's level 
of symptoms and his or her situation.
There is growing interest in the genetic contribution to ADHD, 
especially since recent reports linking ADHD to certain specific genes. 
At least three different genes have been implicated so far, however, so 
the genetics of ADHD remains unclear.
Further research is needed to clarify the genetic contribution to ADHD, 
preferably studies in which children are followed up over time. The 
results would tell much about how the gene or genes responsible for ADHD 
manifest themselves from childhood through adulthood.
The citation will follow.
               The Few. The Proud. The Chosen.
                     markprobe@aol.com
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