The highly simplified model shown in Figure 7.7 is a purely psychological model of ADHD. However, the evidence we have considered shows that ADHD is a disorder that is under strong genetic influences. Genetic risk factors appear to have clear effects on neurotransmitters in the brain (particularly dopamine and norepinephrine). The details of these genetic mechanisms are not yet under- stood and it seems likely that many diverse genes of small effect will be causally related to the development of ADHD. There is evidence for genetic effects on dopamine transporter mechanisms that may be related to the development of ADHD, though once again the magnitude of such effects appears to be too small to provide anything close to a complete account of the origins of the disorder (so genetic effects on the dopamine transporter mechanism will not be sufficient to account for ADHD, though such effects may be one contributory cause of the disorder)