This paper comprises two distinct chapters that explore the nature of son preference in Vietnam, a country that exhibits the second highest sex ratio at birth in the world. Chapter 1 investigates birth-order-specific son preference and the impact of compulsory primary education on son preference using four waves of Vietnamese decennial censuses (1989–2019). The study finds that a desire to have a son at the first birth order—referred to as son-starting behavior—has emerged over time, particularly among small families. By examining the disproportionate increase in education among ethnic minorities in a difference-in-differences framework, triggered by the 1991 compulsory primary education law, the paper finds female primary education leads to a 2.77 pp reduction in son-starting behavior. Although this reduction does not fully eliminate son preference, it significantly mitigates the skewed preference at the first birth order. Chapter 2 examines the association between fertility decline and the sex ratio at birth at the provincial level in Vietnam. Using a Bartik-style instrument based on the variation in the age composition of women, the study finds that the IV estimates are biased, as volatility in the age composition of women violates both exogeneity and exclusion restriction. The chapter concludes that individual-level data may offer more reliable insights into the relationship between fertility and the sex ratio at birth. Together, the paper contributes to a deeper understanding of how son preference is measured and analyzed for causal inference.