The researchers explained that the effects of sunlight could account for these differences. Sunlight helps maintain better sleep patterns and counters the effects of "blue light" from televisions and computer screens which can interfere with sleep. Lack of sleep can cause symptoms similar to ADHD such as irritability and inability to concentrate. The researchers also noted that children may get more exercise in areas where there is more sunlight which improves brain function.
The study is published in Biological Psychiatry.
For more information, please see: