Evidence from cross-sectional data reveals a negative relationship between family income and fertility. This article argues that constraints to intergenerational transfers are crucial for understanding this relationship. If parents could legally impose debt obligations on their children to recover the costs incurred in raising them, then fertility would be independent of parental income. A relationship between fertility and income arises when parents are unable to leave debts because of legal, enforcement or moral constraints. This relationship is negative when the intergenerational elasticity of substitution is larger than one, a case in which parental consumption is a good substitute for children's consumption