The exact cause of ADHD remains unknown. ADHD tends to run in families.  Data from numerous twin studies, adoption and family studies suggest that ADHD is a long lasting disorder with a biological background and an average heritability (the variance explained by additive genetic factors) of around 76%, indicating that familial influences on ADHD are largely genetic. Environmental factors are also likely to play a role either as main causal factors in a few cases or by interaction with genetic risks.

Molecular research has shown that more than one gene are implicated in the expression of the disorder. Recent studies also show that the brain chemical, Dopamine, may play a role in ADHD. Dopamine is an important chemical that carries signals between nerves in the brain, affecting many functions, including movement, sleep, mood, attention and learning.  

Environmental factors e.g. exposure to toxins (lead, PCBs, pesticides) may interfere with the brain development in children and may cause symptoms associated with ADHD. Furthermore, although many parents and adults with ADHD believe that foods with sugar and food additives make their children more hyperactive, this has not been confirmed so far by any study, as a causative factor. Mothers, who smoke cigarettes, consume alcohol or other addictive substances during pregnancy, may have an increased risk for ADHD in their offsprings. Prematurity may be associated with ADHD. Also, brain damage may with be a causative factor for attention deficit disorder in a minority of children.

Using various imaging techniques, researchers have shown a possible link between ADHD and brain structure and function of several areas of the brain with the frontal area likely having the greatest involvement affecting attention and impulse control. Those brain areas involved seem to be responsible for certain executive functions that control the regulation of behavior, working memory, thinking, planning and organizing. The mechanisms by which the disorder persists in some individuals and remits in others with age, has been the subject of interest for many researchers. Jeffrey Halperin proposed the ‘developmental hypothesis’, according to which ADHD is linked to an early-appearing and enduring subcortical dysfunction, while symptom remission is dependent on the extent of maturational changes in executive control. Nonetheless, remission or persistence of ADHD symptoms related to the emerging balance between cortical and sub-cortical function requires further detailed investigation.