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ADHD researchers counter the skeptics by pointing to a growing body of biological clues. For instance, several studies have found that people with ADHD have decreased blood flow and lower levels of electrical activity in the frontal lobes than normal adults and children. In 1990 Dr. Alan Zametkin at the National Institute of Mental Health found that in PET scans, adults with ADD showed slightly lower rates of metabolism in areas of the brain's cortex known to be involved in the control of attention, impulses and motor activity.
Zametkin's study was hailed as the long-awaited proof of the biological basis of ADD, though Zametkin himself is quite cautious. A newer study used another tool -- magnetic resonance imaging -- to compare the brains of 18 ADHD boys with those of other children and found several "very subtle" but "striking" anatomical differences, says co-author Judith Rapoport, chief of the child psychiatry branch at NIMH. Says Zametkin: "I'm absolutely convinced that this disorder has a biological basis, but just what it is we cannot yet say."
What researchers do say with great certainty is that the condition is inherited. External factors such as birth injuries and maternal alcohol or tobacco consumption may play a role in less than 10% of cases. Suspicions that a diet high in sugar might cause hyperactivity have been discounted. But the influence of genes is unmistakable. Barkley estimates that 40% of adhd kids have a parent who has the trait and 35% have a sibling with the problem; if the sibling is an identical twin, the chances rise to between 80% and 92%.
Interest in the genetics of ADHD is enormous. In Australia a vast trial involving 3,400 pairs of twins between the ages of 4 and 12 is examining the incidence of ADHD and other behavioral difficulties. At NIMH, Zametkin's group is recruiting 200 families who have at least two members with ADHD. The hope: to identify genes for the disorder. It is worth noting, though, that even if such genes are found, this may not settle the debate about ADHD. After all, it is just as likely that researchers will someday discover a gene for a hot temper, which also runs in families. But that doesn't mean that having a short fuse is a disease requiring medical intervention.
TRICKY DIAGNOSIS
In the absence of any biological test, diagnosing ADHD is a rather inexact proposition. In most cases, it is a teacher who initiates the process by informing parents that their child is daydreaming in class, failing to complete assignments or driving everyone crazy with thoughtless behavior. "The problem is that the parent then goes to the family doctor, who writes a prescription for Ritalin and doesn't stop to think of the other possibilities," says child psychiatrist Larry Silver of Georgetown University Medical Center. To make a careful diagnosis, Silver argues, one must eliminate other explanations for the symptoms.