This study estimates the causal effect of family size and birth order on educational attainment of individuals in the long run. Following recent literature we use the presence of twins as an instrumental variable for family size and fixed effects model for birth order. The results suggest that in Chile there is a negative relationship between family size and individuals’ educationa achievements. Moreover, it shows a strong significant and positive effect of being a big brother. However, in larger families there is a nonlinear effect in which middle siblings are less favored.