How does delayed childbearing promote child gender diversity? — Causal evidence and mechanism analysis from China

BACKGROUND: In recent years, the age at first birth in China has continued to rise, yet the traditional ideal of having both sons and daughters (child gender diversity) remains deeply ingrained. OBJECTIVE: To examine the causal effect of age at first birth on child gender diversity in China. METHODS: Using pooled cross-sectional data from the Chinese General Social Survey and Chinese Social Survey (N = 45,888), we employ an instrumental variable approach exploiting the 1999 higher education expansion. RESULTS: Delayed age at first birth significantly increases the probability of achieving child gender diversity, with the effect remaining robust across multiple validity checks. Heterogeneity analyses show the effect is universal yet strongest among highly educated groups. Mechanism analysis reveals that a compressed mean birth interval explains over half of the total effect, with full mediation for men but only partial mediation for women. CONCLUSIONS: In China's high-planning fertility environment, delayed childbearing promotes rather than reduces child gender diversity-a pattern opposite to that observed in Western settings. The findings highlight the role of planned fertility sequencing over biological mechanisms and offer targeted insights for fertility support policies.