Although voluminous research has linked nonresident fatherhood to riskier sexual behavior in adolescence, including earlier sexual debut, neither the causality of that link nor the mechanism accounting for it has been well-established. Using data from the National Longitudinal Survey of Youth, 1979--the Young Adult Survey (CNLSY-YA), the present study addresses both questions by comparing the sexual development of siblings discordant for age at father departure from the home and examining results across behavioral (age at first intercourse), biological (pubertal timing), and cognitive (attitudes about childbearing and marriage) sexual outcomes (N = 5,542). Findings indicate that nonresident fatherhood, beginning either at birth or during middle childhood, leads to an earlier sexual debut for girls, but not for boys, an effect likely explained by weak parental monitoring rather than an accelerated reproductive strategy.