Two matched groups of adults - with and without ADHD diagnosis - will perform a battery of computerized tasks assessing sustained attention,working-memory,non-verbal IQ,and decision-making ability. In each group,half of the participants will perform the tasks after receiving a capsule containing methylphenidate (MPH), and the other half will perform the tasks after receiving placebo. Task performance measures will then be compared in order to assess the effect of MPH on these domains.