The seemingly rampant economic selfishness suggested by many recent corporate scandals is at odds with empirical results from behavioural economics, which demonstrate high levels of prosocial behaviour in bilateral interactions and low levels of dishonest behaviour. We design an experimental setting, the 'Big Robber' game, where a 'robber' can obtain a large personal gain by appropriating the earnings of a large group of 'victims'. In a large laboratory experiment (N = 640), more than half of all robbers took as much as possible and almost nobody declined to rob. However, the same participants simultaneously displayed standard, predominantly prosocial behaviour in Dictator, Ultimatum and Trust games. Thus, we provide direct empirical evidence showing that individual selfishness in high-impact decisions affecting a large group is compatible with prosociality in bilateral low-stakes interactions. That is, human beings can simultaneously be generous with others and selfish with large groups. Alos-Ferrer et al. introduce the Big Robber game to study selfish and generous behaviour within the same person. Most people were willing to steal half the earnings of a large group if their personal gain exceeded euro100, but the same people were generous towards individuals.