ADHD tends to run in families. More than 20 genetic studies provide evidence that ADHD is an inherited disorder. At least 2 genes have been associated with the disorder. Most children with ADHD have a close relative who also has it. The National Institute of Mental Health reports that about one-third of fathers who suffered from ADHD during childhood have children with it. Children with affected parents are three times more likely than other children to develop the disorder, and identical twins are both likely to be affected.

Other possible causes and risk factors of ADHD include the following:

Brain structure

Brain chemicals (i.e., neurotransmitters)

Medical conditions

Learning disabilities

Mental health conditions (e.g., conduct disorder, oppositional defiant disorder)

Tourette's syndrome

Computer imaging of the brain structure of people with ADHD sometimes reveals smaller basal ganglia and reduced frontal lobe activity. Basal ganglia, or nerve clusters, are involved in routine behaviors, and the frontal lobes are involved in planning and organizing, attention, impulse control, and inhibition of responses to sensory stimulation.

The neurotransmitter dopamine is involved in controlling emotions and reactions, concentrating, reasoning, and coordinating movement. An abnormally low level of dopamine can cause the three primary symptoms of ADHD: inattention, impulsiveness, and hyperactivity.

Mechanisms involved in the function of dopamine may cause this low level. The fact that stimulants increase levels of dopamine and other neurotransmitters (i.e., epinephrine and serotonin) and help reduce ADHD symptoms suggests that complex interactions between these neurotransmitters are involved in ADHD.

Several medical conditions are associated with ADHD, though ADHD does not necessarily occur as a result of them. Generally, children who experience brain trauma during pregnancy, delivery, or immediately after birth, are at a greater risk for ADHD.

About 50% of children with pediatric autoimmune neuropsychiatric disorder associated with streptococcal (strep) infections (PANDAS) have ADHD.

Alcohol and drug abuse (including nicotine) during pregnancy can cause poor motor and muscular development and sensory impairment; problems with learning, memory, attention, and problem solving; and problems with mental health and social interactions.

Lead poisoning found in infants and children exposed to paint that contains lead has been implicated in ADHD. Specifically, it causes irritability, poor concentration, and distractedness.

The National Institute of Mental Health estimates that 20–30% of the 4,000,000 children in the United States with learning disabilities also suffer from ADHD. Twenty percent to 40% of children with learning disabilities may be prone to recurrent defiance or malicious conduct and may be diagnosed with ADHD and either oppositional defiant disorder (ODD) or conduct disorder (CD).

About 70% of people who suffer from involuntary twitching or spasms (tics) caused by Tourette's syndrome also have ADHD. However, Tourette's is not common among people with ADHD.