To expand on this further: ADHD diagnosis is usually started by teachers filling a questionaire. The questionnaire is a few dosen questions, all of which boil down to "is this boy troublesome?" Questions are geared to behaviors that are typically associated with boys. If the score is high enough, the child is referred for further evaluation. So, girls are under-diagnosed because they score low on "annoying boy" behaviors. Thus girls are heavily under-diagnosed for ADHD. Boys that don't annoy teachers, whether due to conscious effort, due to friendly personality or due to having interest in learning, likewise get under-diagnosed because subconsciously the teachers will assign lower scores for things like general lack of organization because the kid is "good".