Abstract
This paper updates the author's earlier hypothesis that Attention Deficit Hyperactivity Disorder (ADHD) reflects underactivity in Gray's Behavioral Inhibition System. Five areas of research are reviewed: (1) studies using the stop-signal task, (2) studies of errors of commission, (3) a study of inhibition indexed by eye movements, (4) a neuroimaging study of the corpus callosum, and (5) a study on the prediction of response to methylphenidate. Data from the many different dependent variables in these studies are interpreted as supporting disinhibition as a core deficit in ADHD.