 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 . 
