Current scientific consensus shows that AD/HD is principally a genetic,
inherited medical condition. This means that many families have a parent
or close relative who has similar problems. Sometimes a parent may show
the condition more severely than the child and vice versa
(Alban-Metcalfe & Alban-Metcalfe, 2001). Twin studies suggest a 90%
concordance rate in mono-zygotic twins. In other words, if one identical
twin has ADHD there is a 90% chance that the other twin is also affected
(Myttas, 2004; ADDISS, 2004; Knivsberg, Reichelt, & Nodland, 1999).
Further, if there is one child that has ADHD in the family, there is a
30-40% chance that other siblings may also have the disorder (ADDISS,
Prevailing research has shown that individuals with AD/HD compared with
those without AD/HD have ‘under-active’ parts of the brain under
specific test conditions. Modern brain scans (PET and SPECT) have shown
that there is less brain activity, particularly in the frontal area of
the brain. Studies have shown decreased amounts of blood flow to the
striatum and orbital prefrontal brain regions of AD/HD children.
Decreased blood flow is seen more in the right hemisphere than the left.
Diminished flow in these underactive brain centres is caused by reduced
brain activity. These areas of the brain are known to be important in
behaviour inhibition, inattention and incentive learning. In rare
circumstances AD/HD arises without a genetic predisposition. Such cases
include children exposed to significant neurological injuries such as
environmental toxins, alcohol or drug abuse during pregnancy and also
very low birth weight (Barkley, 1992; Flick, 1998; Myttas, 2004; ADDISS,
2004; Knivsberg, Reichelt, & Nodland, 1999).
It is important to remember that AD/HD is not caused by poor parenting
or dysfunctional families. This is not, however, to deny that
environmental factors affect the extent to which the symptoms are
manifested in a given child (Barkley, 1992; Flick, 1998; Myttas, 2004;
Food additives are also being blamed from time to time as a potential
cause of hyperactivity and learning disabilities. However, research has
been unable to support this claim. Although, individuals showed a slight
increase in activity level or inattentiveness when consuming these
substances it was later found that the behaviours were instigated due to
allergic responses by a specific additive that in turn caused discomfort
to the individual and resulted in the observable symptoms. There has
been no evidence that suggests that normal children acquire AD/HD when
they consume such substances or that these substances make their
behaviour considerably worse (Pumfrey & Reason, 1995; Barkley, 1992).