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 observability 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 technique for increasing perceived observability can be replicated whenever public goods solicitations are made in private.H ow can we increase contributions to public goods-to get donors to give more to charity, citizens to vote, households to consume less energy, drivers to carpool, and patients to take all of their antibiotics? One of the best ways is to make contributions more observable (1, 2), as demonstrated by a large body of laboratory experiments (3-9) and a growing body of field experiments (for a review, see ref.2) in a variety of settings, including energy conservation (10), blood donations (11), national park contributions (12), and voting (13).Observability increases contributions to public goods such as voting or charitable giving because observability allows contributions to affect reputations. Individuals who are observed to have contributed can be held in good standing and rewarded in subsequent relationships, either when others are more likely to engage them in a relationship in the first place (this is called partner choice; e.g., refs. 14 and 15) or when others are more cooperative with them during an existing relationship (this is called indirect reciprocity; e.g., refs. 16-21). And, individuals who are observed to not contribute can be held in poor standing.Even subtle cues of observability can increase contributions. In fact, observability can affect contributions when the reputational consequences of one's choice have been entirely eliminated (22,23). An example is eyespots: simply displaying a picture of a face or an abstraction resembling a face increases contributions (24, 25). Such effects imply that the psychology governing our reputations operates at the intuitive level (24)-that is, people do not necessarily deliberate over the reputational gains of every cooperative action, and instead rely on heuristics. Such an intuitive psychology might develop if the heuristics usually work (26,27). For example, if seeing something that looks like a face is usually an accurate indication that someone is watching, then it may pay to give more whenever in the presence of something that looks like a face, even though a clever researcher may exploit this heuristic to induce people into giving a little more in an experiment. Moreover, there...