People contribute more to public goods when their contributions are made more observable to others. We report an intervention that subtly increases the observability of public goods contributions when people are solicited privately and impersonally (e.g., mail, email, social media). This intervention is tested in a large-scale field experiment (n=770,946) in which people are encouraged to vote through get-out-the-vote letters. We vary whether the letters include the message, “we may call you after the election to ask about your voting experience.” Increasing the perceived ob-servability of whether people vote by including that message increased the impact of the get-out-the-vote letters by more than the entire effect of a typical get-out-the-vote letter. This tech-nique for increasing perceived observability can be replicated whenever public goods solicitations are made in private.