 Despite being the most commonly studied and diagnosed mental disorder in children and adolescents, the cause is unknown in the majority of cases. The World Health Organization (WHO) estimated that it affected about 39 million people as of 2013. It affects about 5 – 7 % of children when diagnosed via the DSM-IV criteria and 1 – 2 % when diagnosed via the ICD-10 criteria. Rates are similar between countries and depend mostly on how it is diagnosed. ADHD is diagnosed approximately three times more in boys than in girls. About 30 – 50 % of people diagnosed in childhood continue to have symptoms into adulthood and between 2 – 5 % of adults have the condition. The condition can be difficult to tell apart from other disorders as well as to distinguish from high levels of activity that are still within the normal-range.
