This paper examines relationship between the suitability of land for plough agriculture and the relative value of male and female children in Vietnam. It is hypothesized that a link between geology and son preference operates primarily through household formation rules, which are partially a consequence of the suitability of land for plough agriculture. A quasi-natural experiment measures the causal impact of the sex of a woman’s first-born child on fertility and under-5 mortality of first-borns. Geography fully accounts for the large observed differences across household types in the impact of male first-borns on, respectively, rural women’s fertility under-five child mortality.