||Family structure, particularly sib-ship structure, has a long-term effect on children’s subjective well-being. This thesis examines the causal effect of growing up as an only child on subjective well-being, as measured by self-rated happiness, confidence, and depression. The thesis takes advantage of the exogenous fertility shock of China’s One-Child Policy, which was implemented since 1979. Our results show that being an only child significantly decreases subjective well-being, but this result is mainly driven by rural boys. A tentative explanation for this pattern is provided based on the disparity of son preference in rural and urban China.