A new study of twins suggests that attention-deficit hyperactivity disorder (ADHD) is strongly linked to genes, and that it is less a clear-cut disorder than an extreme form of a trait, like blood pressure, that varies throughout the population. The findings, reported in the June issue of the Journal of the American Academy of Child & Adolescent Psychiatry, suggest doctors should treat ADHD as a condition that varies in severity rather than prescribing a single drug regimen.
ADHD afflicts between 4% and 6% of school-age youth. Children with the condition often fidget, are easily distracted, and impulsively blurt responses in conversation. Past studies have implicated multiple genes--on chromosomes 5, 6, and 11--in ADHD. But questions remained about how important the genetic factors are and whether the disease is a discrete disorder like muscular dystrophy, for example, or a trait requiring treatment only when it reaches a certain threshold.
To find out, Florence Levy of the Prince of Wales and Sydney Children's Hospital, in Australia, and a team of researchers surveyed 1938 families with twins and siblings between the ages of 4 and 12. They asked mothers to rate their children's behavior based on 14 ADHD symptoms laid out by the American Psychiatric Association. The researchers compared the similarity of ADHD symptoms in three separate groups: identical twins, fraternal twins, and siblings. The greater similarities seen in the twins indicated that genetic factors accounted for at least 75% of the differences in ADHD. That's a much larger genetic influence, or heritability, than is seen in many other behavioral disorders, such as alcoholism and schizophrenia. Further analysis indicated that the heritability of ADHD symptoms didn't differ significantly between a subset of children who had five or more of them and the overall group of children. If ADHD were a clear-cut disorder that was either present or not, researchers would expect to find a higher heritability in the group with more symptoms, says Levy. But because they didn't, this suggests ADHD is not a clear-cut disorder and is rather the extreme end of a continuum of behaviors.
This conclusion should promote "rethinking the disorder," says Craig Edelbrock, a developmental psychologist at Pennsylvania State University in University Park. "Current approaches to diagnosing ADHD depend on a somewhat arbitrary cutoff point for the number of symptoms." Thus, conclude the authors, ADHD treatment should be tailored to each case.