People who are impulsive tend to act before thinking. There are many varieties of impulsivity from the premature expression of behaviour before sufficient information is gathered (‘reflection impulsivity’) to the tendency of accepting small immediate or likely rewards as opposed to larger but delayed or unlikely rewards (‘choice’ impulsivity). It is also on occasion associated with aggression. In conceptual terms it is helpful to categorise impulsivity into ‘stopping’ and ‘waiting’ subtypes. In the former case this is assessed by the inhibition or cancellation of a response that has already been started. This is different from ‘waiting’ impulsivity which is best described as a dislike or intolerance of delayed rewards.