The keyword in the question is "diagnosed"! There is a lot of statistical evidence to support the idea that ADHD is diagnosed in more than three times as many school-going boys than girls. (Some studies even put it as four times as many!) But there has never been any rational reason given for this difference, and increasingly the experts dealing with the disorder are coming to accept the idea that the difference is not in the number of children affected by the disorder, but rather in the differences in how ADHD is diagnosed.