The areas of the brain responsible for functions such as attention and memory develop slower in children with attention-deficit/hyperactivity disorder than those of their non-ADHD counterparts, new research suggests.

Scientists from the U.S. National Institute of Mental Health in Maryland and the Montreal Neurological Institute at McGill University found that the parts of the brain responsible for higher cognitive functions develop up to three years slower in children with ADHD.

In their report, published in Monday's online edition of Proceedings of the National Academy of Sciences, the research team estimated that ADHD affects between three and five per cent of schoolchildren.

To reach this conclusion, the scientists measured the thickness of the outer layer of the brain, the cortex, at 40,000 points of the brain of 223 children with ADHD and 223 others who were developing in a typical way. The scans were repeated two, three or four times at three-year intervals. 

The team found that, on average, children with ADHD reached peak cortex thickness when they were 10.5 years old, in comparison to other children at 7.5 years. 

The delay was most pronounced in the brain's lateral prefrontal cortex, which is responsible for controlling attention, memory and high-order motor control. They also found a delay in the temporal cortex, which is responsible for coordinating sensory data with motor functions, such as sound recognition.

One area of the brain developed more quickly in children with ADHD — the motor cortex, responsible for movements. The research team said this finding could account for the fidgety behaviour associated with the disorder.

"Finding a normal pattern of cortex maturation, albeit delayed, in children with ADHD should be reassuring to families and could help to explain why many youth eventually seem to grow out of the disorder," lead researcher Dr. Philip Shaw of the U.S. National Institute of Mental Health said in a statement.

The study said that the difference in brain development is a "delay rather than deviance" and that by the teen years the brains were at similar thicknesses.

According to Dr. Louis J. Kraus, chief of child psychiatry at Rush University Medical Center in Chicago, "What is really important about this study is it shows us there is clearly something biologically driven for children with ADHD."

Kraus, who was not part of the research team, said that with this finding no one can argue that children are making up reasons for their behaviour.

 "We don't know what the meaning is yet, whether it would change any type of treatment, but it is showing that there is something biologically different," Kraus said.

With files from the Associated Press