<p>This paper studies how international marriage migration affects sex ratios at birth in Vietnam. Leveraging a unique context in which the opportunity for Vietnamese women to migrate to wealthier East Asian countries increased the economic benefits of having daughters, we employ a difference-in-differences approach using a demand-driven visa processing change from Taiwan to examine whether Vietnamese households became less likely to have sons. We find a significant reduction in the likelihood of male births in affected provinces, with two fewer boys per hundred girls. We observe a decline for both first- and second-born children, with a stronger effect for the second-born when the first-born is a girl. These findings are consistent with a model incorporating both lexicographic (“at least one son”) and standard son preferences and are inconsistent with either preference alone. This highlights the nuanced role of household preferences in shaping sex selection. Overall, our results underscore the potential of policies that enhance daughters’ economic value to address gender imbalances at birth.</p>