Parental mixed-gender preferences revealed by IVF twin births.

In vitro fertilization (IVF) affects human fertility by 1) increasing the probability of multiple births through the transfer of multiple embryos, 2) raising the proportion of dizygotic twins, which leads to a higher occurrence of one-boy-one-girl twins, and 3) enabling gender selection through preimplantation genetic screening (PGS) when parents have specific gender preferences. While the above-mentioned increase in multiple births is well documented, the impact on the twins' gender composition remains under-explored, partly because 2) and 3) are intertwined. This study uses millions of administrative observations from Taiwan to disentangle the effects of IVF on twin birth rates, gender composition, and parental preferences. Families are grouped by maternal age and family wealth across different periods, enabling a comparative analysis of socioeconomic and temporal factors. By examining the gender composition of first-born twins within these age-wealth-time groups, we can identify and test whether parents have gender preferences for one-boy-one-girl twins. Our empirical evidence reveals a significant increase in IVF adoption, particularly among older and wealthier mothers. The proportion of one-boy-one-girl twins among first-birth twins has risen from 20.42 to 41.86% over the past 30 y, and this proportion increases with wealth ranking, significantly exceeding expected ratios in the absence of gender preference. Parental gender preference contributed to approximately 10.95% of one-boy-one-girl twins among the wealthiest group, revealing a significant impact of parental preference on twin gender composition. These trends raise ethical and social concerns regarding reproductive technologies.