We document that China's One-Child Policy (OCP), one of the most radical approaches to limiting population growth, has produced significantly less trusting, less trustworthy, more risk-averse, less competitive, more pessimistic, and less conscientious individuals. Our data were collected from economics experiments conducted with 421 individuals born just before and just after the OCP's introduction in 1979. Surveys to elicit personality traits were also used. We used the exogenous imposition of the OCP to identify the causal impact of being an only child, net of family background effects. The OCP thus has significant ramifications for Chinese society.