60 Chapter 5
7 Intellectual disability. If an individual’s attention and activity control are
in line with his or her mental age, you should not diagnose ADHD. This
is as true for someone with an intellectual disability as for anyone else.
Thus, if a 10-year-old boy has a mental age of 6, then you should not
diagnose ADHD if his level of attention and activity control is in line with
what you would expect of an average 6-year-old. However, you should
consider diagnosing ADHD if his level of attention and activity control is
at 3-year-old level, that is, well below what you would expect, given his
mental age of 6. While intellectual disability does not necessarily result
in ADHD, it does increase its likelihood. Overall, ADHD is some 10–30
times more common among children and adolescents with intellectual
disability.
Causation
Several lines of evidence from pharmacology, neuroimaging and genetics
suggest that two catecholamine neurotransmitters – dopamine and nora-
drenaline – play a key role in ADHD. However, this evidence is suggestive
rather than conclusive. It is still possible that changes in one or both
of these catecholamines are the result of some other more fundamental
pathophysiology.
Family, adoption and twin studies suggest that genetic factors make a
major contribution, with heritability estimates of around 75%. Evidence
is accumulating that the risk of ADHD varies with polymorphisms of
some of the genes involved in dopaminergic and noradrenergic transmitter
systems, for example, the dopamine D4 receptor gene and the DAT1
dopamine transporter gene. However, these identified genes only account
for a small proportion of the heritability of ADHD, and genome-wide
searches have been negative. What could account for a high heritability
that cannot be explained by currently identified genes? Perhaps there
are still many susceptibility genes of small effect that have not yet been
discovered, but other possible explanations need to be considered. Firstly,
gene–gene interactions may be important, such that several genetic effects
that are individually small combine to produce a major effect in combi-
nation. Secondly, gene–environment interactions may act similarly, with
a gene that has little or no effect by itself producing a major effect when
combined with some specific environment, such as exposure to lead or
some other neurotoxin. Thirdly, ADHD may be caused by many rare genes
with major effects. Although such genes would make ADHD highly herita-
ble within a single family, the effects would be hard to demonstrate when
pooling a lot of different families, each with its own rare gene. Finding rare
but powerful genes may require studies of particularly informative families
that have many affected members.
Though epilepsy and other brain disorders do increase the likelihood
of ADHD (and other psychiatric problems), most individuals with ADHD
have no neurological symptoms or signs: ADHD is not synonymous with