ABSTRACT Epidemiological data point strongly to the period of late adolescence and early adulthood as the critical age during which most individuals initiate engagement in risky behaviors. Two key factors in the initiation of these behaviors are: 1) the individual's propensity to take risks (i.e. how they respond to uncertainty); and 2) the effect of social norms on individual risky decision making. In the previous period of this grant, we focused on the neurobiological underpinnings associated with an individual's response to uncertainty and the neural correlates of risky decision making. In this renewal, we propose to build on these results and address the social dimension of decision making. Using fMRI, we will determine how social messages become intertwined with the elements of uncertainty and value in the brain. The approach is grounded in neuroeconomics, which utilizes neuroimaging to uncover the relationships of hidden variables in the brain that correspond to specific decisions that individuals make. Using paradigms widely studied in experimental economics, we will test competing hypotheses about the manner in which social messages influence these types of decisions by examining the specific neural circuits that mediate the effect of social messages on levels of individual risk taking and cooperation in group settings. The overarching hypothesis is that social messages distort the expected utility of outcomes, and this will be measurable as a shift in brain activity in orbitofrontal-striatal circuits. We hypothesize this effect will be relatively insensitive to the type of message and the type of decision. To test this hypothesis, we propose 4 aims: 1) Determine the neural circuits involved in processing socially salient messages during risky decision making. 2) Determine the extent to which the source of a social message affects the specific neural circuits used to process the message. We will compare the neurobiological effects of peer-to- peer messaging, messages from authority figures and experts, and collective messages. 3) For each of the different messaging sources, we will measure its effect on 3 types of decisions that range from individual, to 2-person, to individual with consequences for others. 4) Determine the developmental trajectory of both the behavioral and neurobiological effect of social messages on risky behavior from adolescence through adulthood.