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Can small groups of individuals efficiently predict population-level behavior? People are notoriously limited in their ability to predict their own future behavior and accurately identify their internal mental states through verbal and written self-reports (Nisbett & Wilson, 1977). Furthermore, explicitly ask-ing participants to reflect on such internal mental states (e.g., “Why do you like this?”) has been shown to alter the outcome and quality of judgments (Wilson & Schooler, 1991). Thus, it is not surprising that public-health media messages selected using traditional focus groups—which rely on these forms of self-report—are also imperfect predictors of population-level responses (Noar, 2006). Recent research has identified neural indicators of individ