Genetic factors Evidence for a hereditary basis for ADHD includes the finding that there is a higher incidence of the disorder in twins who developed from one egg than in twins who developed from two eggs. Also, brothers or sisters of hyperactive children have about twice the risk of having the disorder as do other children. Biological parents of children with the disorder have a higher incidence of ADHD than do adoptive parents of ADHD children. Brain damage It has long been theorized that some children with ADHD received minimal and subtle injury to the brain sometime during their fetal or pre-birth period. Subtle brain injury in the developing child may also occur during pregnancy due to poor maternal circulation and a reduced oxygen supply, or by toxic substances produced in metabolism or from external sources.
Animal studies show that maternal stress during pregnancy affects behavior in offspring.
The same effects may occur in humans. Psychiatric studies show that mothers with high anxiety levels are likely to give birth to babies who are hyperactive. Brain injury may also occur after birth, in infancy, due to infections, inflammation and physical trauma. Efforts to discover structural or functional changes in the brain of ADHD patients have yielded mixed results. For example, a reduction in signals sent from the frontal lobe to control activity would result in the hyperactivity seen in the children.