Although the majority of people value the idea of helping others, they often take no particular action. In two field studies we investigated the impact of differently justified requests for spontaneous charity donations and for antisocial behavior like stealing. In the experiments, unwatched stands with cookies and money jars were placed on a crowded city square with one of three different notes: (1) detailed prosocial justification, (2) general justification or (3) no justification. After testing almost 500 participants, we show that mere general arguments can both increase prosocial behavior and decrease antisocial behavior. Additionally, detailed prosocial justification augments generosity, causing people voluntarily to pay more than required. We conclude that prosocial (compliance with request) and antisocial (stealing) behavior is guided by automatic processes that track that there is any reason for the request, while generosity is guided by reflective assessment of the justification of the request.