As social mammals, being in a group signals a state of relative security. Risk‐taking behavior in other social mammals formed the basis for our prediction that the mere physical presence of others, absent any social interaction, would create a psychological state of security that, in turn, would promote greater risk‐taking behavior. We investigated whether, why, and when the mere physical presence of others affects risk‐taking behaviors in three contexts: acceptance of greater financial volatility, attitudes toward risky gambles, and actual gambling behaviors. Results indicate that people in the mere physical presence of others make riskier decisions than people making identical decisions alone, and that feelings of security were the psychological mechanism behind this effect. Our results also suggest that the effect is contingent on whether people are surrounded by others who belong to the same social group. A meta‐analysis across all studies presented in this research reveals a highly reliable mere‐presence effect. Together, these results demonstrate that the mere physical presence of others can have a potent impact on risk‐taking behaviors.