The effect of a sibling on the first-born child’s health: evidence from two-child families in China

The first-born child's quality may be affected by a younger sibling in a family based on the quantity-quality trade-off theory. Using data from the China Health and Nutrition Survey, we examine the causal effect of having a younger sibling on the health of the first-born child aged 2–12 in China. We use instrumental variables to address the potential endogeneity of having a younger sibling in the extended regression model. We found that having a sibling significantly decreases the height-for-age z-scores of the first-born child, and the greater age gap may alleviate the effect. Further analysis shows that the effect is particularly strong for the pre-school child under 6 years old and the child in a low-income family or the rural area. A sibling influences the first-born child's health by dietary pattern, physical activities, and medical services utilization. The robustness checks, based on individual fixed-effects model and propensity score matching approach, validate our findings, which suggest that future preventive intervention on the deterioration of first-born child’s health during the implementation of the universal two-child policy.