In this paper I seek to identify and estimate the causal effect of fertility on family income. I analyze the endogeneity of the decision to have a second child with a sample of twins at first birth (as in Rosenzweig and Wolpin, 1980). And the decision to have a third son is addressed with an instrumental variables technique: the sex composition of the first two children is a random event and if both children are of the same sex, it exogenously increases the likelihood of parents having a third son (as in Angrist and Evans, 1998). The results suggest that the second child would increase total household income, while income per capita remains constant. For the third child, the effect appears to be negative, although the estimation is not precise.