Although the root cause of ADHD is unknown, there are some factors that may increase the risk that someone will be afflicted with the disorder. These include:

• Heredity: Studies indicate that 25 percent of the close relatives in the families of children with ADHD also have the disorder. This is much higher than the rate of ADHD in the general population which is between 3 and 5 percent.
• Sex: Boys are more likely than girls to be diagnosed with ADHD, although there is some controversy over whether this represents an actual difference in the incidence of ADHD between the sexes or differences in diagnosis rates.
• Brain injury: Children who have had a brain injury such as a sever concussion or brain trauma appear to be more likely to develop ADHD.
• Exposure to chemicals: Several studies have linked exposure to chemicals as a fetus or child to ADHD. Prenatal exposure to cigarettes, alcohol, or other toxins such as polychlorinated biphenyls (PCBs) may reduce the ability of brain cells (neurons) to produce certain neurotransmitters. Childhood exposure to lead and PCBs have already been demonstrated to be associated with behavioral problems similar to ADHD.