The study, published in the Journal of the American Academy of Child and Adolescent Psychiatry, confirms hyperactive and impulsive behaviors do decrease with age, while inattention increasingly becomes predominant.

Two-thirds of children with ADHD continue to exhibit significant levels of inattentiveness and impairment into adolescence, despite the differences between treatment approaches and cultures between the United States and Finland, the study said.

University of California, Los Angeles, researchers, led by Susan Smalley, looked at ADHD teens in Finland that were part of a long-term study begun prenatally in l986 by the Imperial College London and the University of Oulu, Finland.

The ADHD population in northern Finland exhibited two genes associated with ADHD known as DBH and DRD2. These genes affect the regulation of dopamine -- a neurotransmitter involved in attention, motivation and emotion. Although the researchers say the genes likely account for very little of the genetic variation underlying ADHD, the findings further support the involvement of the dopamine pathway in the disorder.