Although the causes of attention deficit hyperactivity disorder (ADHD) are not fully understood, increasing numbers of experts believe that changes in brain structure are a leading cause of the disease. Brain scans have shown alterations of neural pathways that normally carry messages transmitted by the neurotransmitter dopamine. Normally, these messages reach the brain and affect attention, movement and motivation. However, impairment to these neural pathways may prevent the messages from reaching the brain, leading to inattention, lack of planning, increased impulsivity of behavior and lack of motor control. 

Other brain scans have found that patients with ADHD have lower levels of metabolic activity in the areas of the brain that dictate attention, social judgment and movement. More recent imaging studies have also shown brain development that is structurally normal in children with ADHD, but delayed by up to three years.

Additional factors that may contribute to the development of ADHD include:

• Genetics and heredity. ADHD tends to run in families. Studies have shown that up to one-quarter of the close relatives of a child with ADHD are likely to have the disorder themselves, according to the National Institute of Mental Health (NIMH). Identical twins usually both have the disorder. Many genes probably influence ADHD. Studies show that children with ADHD are more likely to have certain variation in the DRD4 genes (genes that makes a chemical in the brain called dopamine), influence ADHD.
  
  
• Maternal exposure to chemicals during pregnancy. Pregnant women who smoke, abuse alcohol and drugs, and are exposed to certain environmental poisons are more likely to give birth to children who are later diagnosed with ADHD. Environmental poisons associated with ADHD include polychlorinated biphenyls [PCBs], which have been banned in the United States, but remain in the environment. They are found in high levels in breast milk, which attracts the contaminants due to its high fat content.
  
  
• Exposure to chemicals during childhood. Young children who are exposed to environmental toxins such as PCBs or lead (found mainly in paint and pipes of older buildings) are at increased risk for developmental and behavioral problems associated with ADHD symptoms.
  
  
• Gender. Boys are three times as likely to develop the condition as girls, according to the NIMH.

Earlier theories suggested that minor head injuries or brain damage caused by infections or birthing complications might be responsible for ADHD. However, experts have not found evidence to support this hypothesis. Theories that too much or too little sugar may cause ADHD also appear to be unfounded and have been disproved in studies. Sugar can make children sluggish rather than hyperactive.