Double take: The effect of sibling sex composition on women’s schooling, earnings, and labor supply

• This study estimates the effect of sibling sex composition on human capital and labor market outcomes using dizygotic twins.
• Women born with male co-twins versus female co-twins have increased rates of high school completion and higher earnings.
• No statistically distinguishable sibling sex composition effects are detected for men.

Understanding the role of the family in the production of human capital is a salient question in economics. Using a twin research design that exploits exogenous gender variation in dizygotic twins, this paper investigates the effect of sibling sex composition on schooling, earnings, and labor supply. Women born with a male co-twin have higher earnings and increased rates of high school completion than women born with a female co-twin. Men born with a female co-twin, on the other hand, have outcomes that are statistically indistinguishable from zero. Family attributes provide a limited explanation of the sex composition effect.