The Stop Signal Test (SST) is a measure of response inhibition. Difficulties of response inhibition in ADHD have been reported by 82% of studies that have examined it. Response inhibition has been posited as an endophenotype for ADHD and shown to distinguish a group of children with ADHD from one with ADHD symptoms and one with conduct disorder. In the SST, participants will perform a choice reaction time task and will be instructed to respond as quickly and accurately as possible. On a portion of trials (e.g. 25%) a stop signal (a tone) will be presented and participants will be required to restrain their motor response. The delay between the go stimulus (the primary task) and the onset of the stop signal varies to determine the participant's ability to stop an already initiated motor response. This individualized stopping latency varies as a function of impulsivity. The stop-signal reaction time (SSRT) provides a measure of the ability to inhibit an already initiated response.