<p>Using China’s family planning policies as natural experiments, we examine the long-term effects of sibship size on adults’ mental health and subjective well-being. We find that having more siblings negatively affects mental health and subjective well-being in adults, increasing the likelihood of depression and psychological distress and lowering individuals’ life satisfaction and confidence about the future. These effects are more prominent among females and those with lower early-life socioeconomic status. Interestingly, being the only child also worsens mental health in adults. We argue that, in the case of only children, the detrimental effects of the “only-child disadvantage” on mental health and subjective well-being may dominate the benefit from the “quantity-quality tradeoff.” We find supportive evidence that adults without siblings have better physical health but are less agreeable and more neurotic than those with siblings. Together, these seemingly contradictory findings reconcile the theory of quantity-quality tradeoff and the studies on the only-child disadvantage and advance our understanding of the impacts of sibship size on mental health and subjective well-being.</p>