Why do men in the United States today complete less schooling than women? One reason may be gender differences in early self-regulation and prosocial behaviors. Scholars have found that boys' early behavioral disadvantage predicts their lower average academic achievement during elementary school. In this study, I examine longer-term effects: Do these early behavioral differences predict boys' lower rates of high school graduation, college enrollment and graduation, and fewer years of schooling completed in adulthood? If so, through what pathways are they linked? I leverage a nationally representative sample of children born in the 1980s to women in their early to mid-20s and followed into adulthood. I use decomposition and path analytic tools to show that boys' higher average levels of behavior problems at age 4 to 5 years help explain the current gender gap in schooling by age 26 to 29, controlling for other observed early childhood factors. In addition, I find that early behavior problems predict outcomes more for boys than for girls. Early behavior problems matter for adult educational attainment because they tend to predict later behavior problems and lower achievement.