Using rich data on familial background of US mutual fund managers, this paper sheds light on the formation of risk preferences by investigating birth order effects. Consistent with sensation-seeking behavior, we find that managers who are born later in the sibling hierarchy take more risks but perform worse relative to lower-birth-order managers. Later-born managers take more extreme style bets, hold more lottery stocks, churn their portfolios more, and engage in more civil and regulatory violations. These birth-order effects are more pronounced when parental resources are limited and age spacing is lower between siblings, suggesting sibling rivalry as a potential mechanism.