Possible Causes

Brain Structures. Research using advanced imaging techniques shows there is a difference in the size of certain parts of the brain in children with ADHD compared to children who do not have ADHD. The areas showing change include:

  • The prefrontal cortex. The prefrontal cortex is located in the front of the brain. It is thought to be the brain's command center. It regulates the brain’s ability to block certain responses. Numerous imaging studies have indicated that the prefrontal cortex of the brain in people with ADHD may be less active than in those without the disorder.
  • The caudate nucleus and the globus pallidus. The caudate nucleus and globus pallidus, located near the center of the brain, speed up or stop orders coming from the prefrontal cortex. A major 2002 study reported that it was smaller than average in young children with ADHD, but tended to normalize as children got older. Abnormalities in these areas may impair a person's ability to stop certain actions, resulting in the impulsivity typical of people with ADHD.
  • The cerebellum. The cerebellum is the area above the brain stem. This area helps control muscle tone and balance, and synchronizes muscle activity. An important 2002 study reported that this area tends to be smaller in children with ADHD compared to those without the condition.

Brain Chemicals:. Abnormal activity of certain brain chemicals in the prefrontal cortex may contribute to ADHD. The chemicals dopamine and norepinephrine are of special interest. Dopamine and norepinephrine are neurotransmitters , or chemical messengers, that affect both mental and emotional functioning. They also play a role in the "reward response." This response occurs when a person experiences pleasure in response to certain stimuli (such as food or love). Studies suggest that increased levels of the brain chemicals glutamate, glutamine, and GABA -- collectively called Glx -- interact with the pathways that transport dopamine and norepinephrine.

Nerve Pathways. Another area of interest is a network of nerves called the basal-ganglia thalamocortical pathways. Abnormalities along this neural route have been associated with ADHD, Tourette syndrome, and obsessive-compulsive disorders, all of which share certain symptoms.

Genetic Factors

Genetic factors may play the most important role in ADHD. The relatives of ADHD children (both boys and girls) have much higher rates of ADHD, antisocial, mood, anxiety, and substance abuse disorders than the families of non-ADHD children. A study reported that 90% of children with a diagnosis of ADHD shared it with their twin.
Genetic Factors Regulating Dopamine and Advantages in Early Man. Most of the research on the underlying genetic mechanisms targets the neurotransmitter dopamine. Variations in genes that regulate specific dopamine receptors have been identified in a high proportion of people with addictions and ADHD. Such genes have been associated with novelty seeking and extroversion. Some experts theorize that the genetic variants may have first appeared thousands of years ago, and affect as many as half of ADHD children. Furthermore, the genetic variations may have offered some benefits to their early carriers. In such people, a genetic predilection for novelty-seeking and risk-taking may have supplied an advantage in reproduction, mating, hunting, and achieving dominance.
Genetic Resistance to Thyroid Hormone. About 50% of adults and 70% of children with a genetic resistance to thyroid hormone, essential for normal brain development, have ADHD. People who have this condition appear to have a more severe form of ADHD. The thyroid disorder is not a common cause of ADHD. Only those with a family history of thyroid disease are at risk.   

Pregnancy and ADHD

ADHD is often associated with problem pregnancies and difficult deliveries. If a women smokes during pregnancy, a genetically susceptible child is at higher risk for ADHD. Some studies also suggest that an increased risk also exists in children of women who were exposed during pregnancy to environmental toxins, including dioxins and polychlorinated biphenyls (PCBs).

Dietary Factors

Infant malnutrition is a strong risk indicator of ADHD. Even if children receive enough food later on, infants who suffer from malnutrition may develop behavior problems, the most prevalent being attention-deficit disorder.
Deficiencies in Zinc and Essential Fatty Acids. Several dietary factors have been researched in association with ADHD, including sensitivities to certain food chemicals, deficiencies in fatty acids (compounds that make up fats and oils) and zinc, and sensitivity to sugar.
Some studies have found an association between deficiencies in certain fatty acids and ADHD. Other research reports an association between zinc deficiencies and ADHD. Zinc aids in the breakdown of fatty acids, which affects dopamine, the neurotransmitter likely to be involved with ADHD.
No clear evidence has emerged, however, that implicates any of these nutritional factors in ADHD.

  • Review Date: 3/5/2007
  • Reviewed By: A.D.A.M. Editorial Team: Greg Juhn, M.T.P.W., David R. Eltz, Kelli A. Stacy. Previously reviewed by Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital (12/14/2006).

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