This paper examines the effect of birth order on educational attainment in the United States and the underlying mechanism producing these effects. Using a family fixed effects model, we find negative birth order effects on educational outcomes. However, this effect varies depending on the household's income, being the strongest for households with the highest income and diminishing as households' income decreases. In addition, we show that the timing of income across childhood is important for completed education, as the largest gap in educational attainment between siblings emerges between those who were born and spent their early childhood in wealthier households.