Assigning a subjective value to a contribution to a public good often requires reflection. For many reasons, this reflection may be put off, reinforcing the underprovision of public goods. We hypothesise that nudging individuals to reflect on whether to contribute to a public good leads to the formation of issue-specific altruistic preferences. The hypothesis is tested in a large-scale field experiment on blood donations. We find that an active-decision intervention substantially increases donations among subjects who had not previously thought about the importance of donating blood. By contrast, contributions of individuals who had previously engaged in such reflection are unchanged.