LOS ANGELES, Jan. 28 (UPI) -- A study of attention-deficit hyperactivity disorder in Finnish children found half with the disorder showed cognitive deficits.

The study, published in the Journal of the American Academy of Child and Adolescent Psychiatry, also found children with cognitive deficits don't show increased levels of inattention or hyperactivity when compared with children diagnosed with ADHD.

Study leader Susan Smalley of the University of California, Los Angeles, said the study findings brought "to light the necessity of engaging in new ways of thinking about ADHD.

"Certainly it is a valid disorder in terms of its diagnosis; there are relatively similar prevalences around the world," Smalley said in a statement.

"But the predisposition to ADHD is an extreme on a normal distribution in attention and activity level, much like diabetes and glucose tolerance, or dyslexia and reading disability."

Smalley advised examining more carefully what environmental pressures may be leading to impairment "instead of broadening our diagnostic classifications even further."

The study, which tracked children 9,432 children from the early fetal period to 16 to 18 years of age, found the estimated prevalence of ADHD among children in northern Finland was 8.5 percent with a male-female ratio of 5.7 to 1.