We document that China's One-Child Policy, 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 One-Child Policy's introduction in 1979. Surveys to elicit personality traits were also used. We use the exogenous imposition of the One-Child Policy to identify the causal impact of being an only child, net of family background effects. The One-Child Policy thus has significant ramifications for Chinese society.