Mailing It In? Results from a Field Experiment Encouraging Voting by Mail in a 2020 Primary

The ability to cast a mail ballot can be an important safeguard to the franchise. But because there are often additional procedural protections to ensure that a ballot cast in person is counted, voting by mail can also jeopardize people's ability to cast a ballot that is counted. An experiment carried out during the COVID-19 pandemic illustrates both of these forces. Philadelphia city officials randomly sent approximately 47,000 Philadelphia voters postcards encouraging them to apply to vote by mail in the lead-up to the June 2, 2020 primary election. While the intervention increased the share of Philadelphia voters who cast a mail ballot by 0.4 percentage points (p=0.017)--or 3%--many of these additional mail ballots counted only because a last-minute policy intervention allowing mail ballots postmarked by Election Day to count.