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Since the early 1990’s researchers have been looking at brain size difference in children with Attention-Deficit/Hyperactivity Disorder (ADHD) as compared to children without ADHD. Zametkin et al (1990) demonstrated the possibility that actual brain size might be different in individuals with ADHD. The topic was also the focus of researchers at the American Academy of Neurology meetings in 1999.

Different reasons have been suggested for the difference in brain size. Some relate to how the brain may respond to medications. This led some to criticize the studies. Some suggested that medication effects, not actual differences in brain size, was being measured. The study reported in this month’s Journal of the American Medical Association is unique because it compares brain size of children with ADHD who have been on medication and those with ADHD but not on medication with children who do not have ADHD.

The sample

152 children and adolescents with ADHD, ranging in age from 5-18, were compared with 139 age and gender-matched controls. Data was collected over a 10-year period from 1991-2001 in the National Institute of Mental Health sponsored study. Researchers collected 544 anatomic magnetic resonance images (MRI’s) , or pictures of the brain’s of the children involved in the study.


  1. Children with ADHD (in both the medicated and non-medicated conditions) had smaller brain volumes. This was found in the white matter of the brain and in the cerebellum , a region at the base of the brain that is associated with muscle tone, balance, and other more basic, non-thinking, functions.
  2. Though the difference in the sizes of the brain reach statistical significance (p.<.004 meaning probability of the effect happening by chance is less than .004 ) the size difference is not so large that one could use brain size as a diagnostic indicator.
  3. A region of the brain called the caudate nucleus was found to be smaller in younger children with ADHD but difference between children with ADHD and those without ADHD disappeared in adolescence.
  4. Significant gender differences were not found.
  5. Brain size differences correlated with the parent and clinician-rated severity of the ADHD sample, Children with more severe symptoms showed smaller brain size.

For more information

Journal of the American Medical Association, October 2002, pages 1740-1748
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