We exploit the randomized assignment of lottery prizes in a large administrative Swedish data set to estimate the causal effect of wealth on stock market participation. A $150,00 0 windfall gain increases the stock market participation probability by 12 percentage points among prelottery nonparticipants but has no discernible effect on prelottery stock owners. A structural life cycle model significantly overpredicts entry rates even for very high entry costs (up to $31,0 00). Additional analyses implicate pessimistic beliefs regarding equity returns as a major source of this overprediction and suggest that both recent and early-life return realizations affect beliefs.