There are strong reasons to assume that early onset of puberty accelerates coital debut among adolescent girls. Although many studies support this assumption, evidence regarding the putative causal processes is limited and inconclusive. In this research, longitudinal data from the 1986 Northern Finland Birth Cohort Study (N = 2,596) were used to address three theoretical explanations: (a) a direct effect premised on biological processes, (b) a mediated path based on social psychological processes, and (c) a spurious effect derived from the evolutionary theory of socialization. In support of the social psychological pathway, the negative association between age at menarche and coital status at age 15 was almost fully mediated by differential social exposure—an empirical construct measuring involvement in high-risk social contexts.