Family, twin, and adoption studies provide evidence
that ADHD has a genetic component.
Heritability
has been estimated at 76%.Metaanalyses
of candidate-gene association studies
have shown strong associations between ADHD
and several genes involved in dopamine and
serotonin
pathways.20 Multiple genes, each with
a small effect, may together mediate genetic
vulnerability.
Nongenetic factors (e.g., maternal
smoking during pregnancy or exposure to environmental
lead or polychlorinated biphenyls)
may also interact with genetic predisposition in
the pathogenesis of ADHD.