Multiple accounts of adolescent impulsivity.

Adults have been concerned about adolescent impetuosity for centuries. The rashness of youth has been blamed for many teenage indiscretions, from juvenile delinquency to unprotected sex and substance use. At the turn of the 21st century, research on the development of self-control in adolescence added functional brain imaging as a complement to behavioral tasks and self-report questionnaires, but the essential question has remained the same: Why do teenagers have so much difficulty controlling their impulses? As a report in PNAS by van den Bos et al. (1) attests, the question continues to be of interest. Although we discuss some potential limitations, the study these authors describe adeptly and cleverly incorporates the broad armamentarium of tools, including self-report, behavior, brain function, and both structural and functional connectivity measures, to move us toward a more complete answer to the question. It is important to put the contributions of van den Bos et al. (1) in historical context. Ten years ago, the pat explanation for adolescent impulsivity was the immaturity of the prefrontal cortex. Adolescents behave impulsively, it was thought, because the brain circuitry necessary to exert top-down control over urges originating in the limbic system was still developing. Studies of brain anatomy revealed important changes over the adolescent decade in both gray and white matter volumes in prefrontal regions, suggesting that synaptic pruning and myelination were enabling more efficient and more effective self-regulation. As adolescent brain science progressed, scientists interested in adolescent self-regulation began to turn their attention to regions other than the prefrontal cortex. One possibility, some suggested, was not simply that cognitive control was immature, but that this immaturity was accompanied by a temporary intensification of urges to pursue novel and rewarding experiences (2, 3). This so-called “dual systems” or “maturational imbalance” perspective on adolescent decision-making has not gone uncriticized (4), but it remains a widely used theoretical model guiding the study of adolescent risk-taking. More specifically, we and others have proposed that adolescents’ disposition toward risk is because of a maturational imbalance between a brain network involved in deliberative, planful, and goal-directed behavior and one involved in affective processes, including the anticipation and valuation of incentives (2, 5). Shortly after puberty, the affective processing system undergoes rapid development, producing increased sensitivity to (and motivation for) reward that declines through late adolescence and into the early 20s. In contrast, structures of the cognitive-control network that inhibit impulses and direct motivation toward goal-relevant behaviors show continued gains into the 30s. If adolescent risk-taking is the byproduct of an easily aroused incentive-processing system and a still-immature cognitive-control system, it is reasonable to ask how important the contributions of each are to teenagers’ impulsive behavior. Many risky situations force a choice between taking a gamble to receive a highly salient immediate reward (e.g., the sensation of unprotected sex) and waiting for a safer but less-rewarding one (e.g., using a condom). Is choosing the immediate reward because of teenagers’ difficulties regulating their desires, or is it because their desires are especially intense? Although the authors frame the problem in slightly different terms, distinguishing between these two accounts is the focus of the van den Bos et al. (1) paper. It is an important question, but it is an exceptionally challenging one because simply observing an individual’s behavior does not provide a clear-cut answer. Consider, for example, a child who has been given the famous Marshmallow Test (6), in which he is asked to choose between receiving one marshmallow immediately or waiting for two. Does the child who chooses the delayed reward of two marshmallows have especially strong self-control, or is he just not particularly fond of marshmallows? Observing the child cannot answer the question. Perhaps looking into his brain can. To address this issue, van den Bos et al. (1) use an intertemporal choice task in which individuals must choose between a small monetary reward given sooner and a more generous one given later. As we and others have reported in previous studies (7), van den Bos et al. (1) find that adolescents are more likely than adults to opt for smaller rewards sooner (SS) than larger ones later (LL). The authors posit that the extent to which individuals are inclined toward SS choices indexes impatience, which they view as one of three aspects of impulsivity (the other two are acting without thinking and sensation-seeking). There is some debate about how best to define impulsivity and its underlying factors (8), and to us it makes more sense to view impatience not as a third aspect of impulsivity, but as the byproduct of poor self-control and high reward sensitivity (in this framing, impatience can be either hasty or considered). Nevertheless, in a realm of inquiry where nomenclature is muddy, the distinction between volitional processes that invoke