This paper develops a theory of fertility that offers an explanation for the persistence of poverty within and across countries. If educated individuals have a comparative advantage in raising educated children then parental fertility choice is shown to give rise to a poverty trap, in which the poor choose high fertility rates with low investment in child quality. Moreover, the impact of child quality choice on economic performance is amplified by the diluting effect of higher fertility on physical capital accumulation. The theory proposes insights regarding the effects of inequality, globalisation and life expectancy on economic growth and demographic transitions.