Attention deficit disorder (ADD ADHD) is one of the most researched areas in child and adolescent mental health. However, much about ADHD -- including the precise cause of the disorder -- is still unknown.

We do know that ADHD is a brain-based biological disorder. Brain imaging studies using PET scanners show that brain metabolism in children with ADHD is lower in the areas of the brain that control attention, social judgment, and movement.

Brain Basics

The image on the left shows differences between an adult with ADHD (right) and a non-ADHD brain (left). (The purple halo surrounding the brain image is an image artifact and not part of the brain.)

Scans and other imaging research have also shown that the brains of children with ADHD differ fairly consistently from those of children without the disorder.

Several brain regions and structures (pre-frontal cortex, striatum, basal ganglia, and cerebellum) tend to be smaller. Overall brain size is generally 5% smaller in affected children than children without ADHD. While this average difference is observed consistently, it is too small to be useful in making the diagnosis of ADHD in a particular individual.

The Chemical Connection

There are also chemical differences in the ADHD brain. Low levels of dopamine (a brain chemical), which is a neurotransmitter (a type of brain chemical), are found in children with ADHD.

Sweet News

At one time, it was believed that refined sugar and food additives made children hyperactive and inattentive. As a result, parents were encouraged to stop serving children foods containing artificial flavorings, preservatives, and sugars.

However, after studying the data, researchers from the National Institutes of Health (NIH), the federal agency responsible for biomedical research, held a major scientific conference to discuss the issue in 1982. These scientists concluded that the restricted diet only seemed to help about 5 percent of children with ADHD, mostly either young children or children with food allergies.

In Your Genes

Available evidence suggests that ADHD is genetic.

• Children who have ADHD usually have at least one close relative who also has ADHD.

• And at least one-third of all fathers who had ADHD in their youth bear children who have ADHD.

• The majority of identical twins share the trait.

At the National Institutes of Health, researchers are also on the trail of a gene that may be involved in transmitting ADHD in a small number of families with a genetic thyroid disorder.