Parents invest differently in sons and daughters. This paper formally re-examines the application of the Trivers Willard hypothesis to humans. Differential fecundity and assortive matching in marriage are necessary for parents to invest differently in the skills of their sons and daughters. Unlike the Trivers Willard hypothesis, better endowed parents do not necessarily invest relatively more on the survival of their sons relative to worse endowed parents. Parents invest more on the health of the child that they value higher. They do not necessarily invest more on the skills of the child that they value higher. The theory generates endogenous population growth.