The Big Five predict numerous preferences, decisions, and behaviors—but why? To help answer this key question, the present research develops the sociocultural norm perspective (SNP) on Big Five prediction—a critical revision and extension of the sociocultural motives perspective. The SNP states: Agreeableness, Extraversion, and Conscientiousness predict outcomes positively if those outcomes are socioculturally normative. Openness, by contrast, predicts outcomes negatively if they are socioculturally normative. Moreover, the SNP specifies unique mechanisms that underlie those predictions. Two mechanisms are social (social trust for Agreeableness, social attention for Extraversion) and two are cognitive (rational thought for Conscientiousness, independent thought for Openness). The present research develops the SNP by means of three large-scale experiments (Ntotal = 7,404), which used a new, tailor-made experimental paradigm—the minimal norm paradigm. Overall, the SNP provides norm-based, culture-focused, and mechanism-attentive explanations for why the Big Five predict their outcomes. The SNP also has broader relevance: It helps explain why Big Five effects vary across cultures and, thus, dispels the view that such variation threatens the validity of the Big Five. It suggests that the psychology of norms would benefit from attention to the Big Five. Finally, it helps bridge personality, social, and cross-cultural psychology by integrating their key concepts—the Big Five, conformity, and sociocultural norms.