In this paper, we propose a decision framework where people are individually asked to either actively consent to or dissent from some pro-social behavior. We hypothesize that confronting individuals with the choice of whether to engage in a specific pro-social behavior contributes to the formation of issue-specific altruistic preferences, while simultaneously involving a commitment. The hypothesis is tested in a large-scale field experiment on blood donations. We find that this "active-decision" intervention substantially increases the actual donation behavior of people who had not fully formed preferences beforehand.