the dynamic developmental theory (ddt) has benefited from the insights of the commentators, particularly in terms of the implications for the proposed steepened delay gradients in attention-deficit/hyperactivity disorder (adhd). the introduction of modified memory processes as a basis for the delay gradients improved the links to aspects of adhd. however, it remains unclear whether the hyperactive-impulsive and inattentive subtypes are separate subgroups or may be explained as different outcomes of the same genetic factors and thus explicable by the same principles. the ddt suggests that altered reinforcement and extinction processes define an endophenotype in adhd that can be related dimensionally to inattention, hyperactivity, and impulsivity. the relation between the suggested endophenotype, characterized by changes in basic learning mechanisms, and other endophenotypes characterized by delay aversion or response disinhibition, needs to be tested in future studies.