This study also represents an important step toward integrating advances in cognitive models of executive control with clinical psychology. The concept of executive control, particularly response inhibition, is central to many theories of psychopathology, including ADHD (Barkley, 1997; Nigg, 2000, 2001) and substance use disorders (Garavan & Stout, 2005; Giancola et al., 1998). Our finding that the link between behavioral disinhibition and response inhibition is almost entirely genetic extends previous behavioral findings to a new level. Understanding the underlying biological processes that link behavioral disinhibition, measured with assessments of real-life behavior, and response inhibition, measured with cognitive laboratory tasks, promises to provide new insights into the nature of externalizing spectrum disorders. In this respect, response inhibition may be a valuable endophenotype for future neuropsychiatric and molecular genetic studies of disinhibited behavior problems. One cautionary note is that the usefulness of response inhibition as an endophenotype will depend in large part on how it is measured. In particular, using individual tasks is unlikely to be fruitful. As the factor loadings for the tasks demonstrate, although the latent response inhibition factor is
