Vote-buying and vote-selling obstruct the democratic process, yet they remain pervasive in many developing democracies. Researchers asked voters in the Philippines to make a simple, unenforceable promise not to accept money from politicians or to promise to vote according to their conscience, even if they do accept money, to test the impact of promises on voters’ behavior. A majority of respondents made promises not to sell their votes. Researchers found that the promise significantly reduced vote-selling, cutting the number of people who sold their votes by 11 percentage points in the smallest-stakes election, but was not effective in the mayoral election with higher pay-outs. These results suggest that simply asking voters to promise not to sell votes can help reduce vote-selling in elections where vote-buying payments are typically small.