Prior to the early 1980s, American men graduated from high school and college at higher rates than American women. Since then, women have comprised a growing majority of high school and college graduates. This growing female advantage in educational attainment carries significant implications for labor markets, marriage markets, fertility and family formation, and child well-being. It also is of consequence for racial/ethnic and socioeconomic inequality: The gender gap in educational attainment is largest among minorities and the poor. Extant labor market and social accounts explain 30-60% of the gap, leaving up to 70% unexplained. This dissertation proposes a new, but complementary, explanation. Drawing upon newly-available data from the Children of the NLSY79, which tracks children born in the 1980s until 2010, part one of the dissertation shows that boys' higher average level of early childhood behavior problems explains 15-25% of their lower level of educational attainment compared to girls. Introducing the Early Childhood Longitudinal Study-Birth Cohort data, the second and third parts of the dissertation compare children born in the 1980s and 2000s to examine whether the gender difference in behavior problems -- like that in educational attainment -- has become most widespread among minorities and low-income Americans. Findings reveal that, by the 2000s, the gender gap in early childhood behavior problems had spread throughout a wide cross-section of minority children and children from low-income families. The behavior gap emerged even between the black and Hispanic and poor boys and girls with the lowest mother-rated behavior problems. Analysis of potential mechanisms shows that growing gender differences in observed demographic, family, and health factors account for 20%-100% of the growth of the gender gap in behavior problems at different points in the behavioral distribution. Some, but not all, of the growth of the gender gap in behavior problems is accounted for by observed population level trends. Some of the growth of the gap in behavior may be due to perceptions of boys' worsening behavior. Taken together, results predict a widening gender gap in attainment in the coming decades, especially among minority and poor children.