The main reason for the increased use of mail-in ballots in 2020 was the COVID-19 pandemic, prompting states to expand access to reduce health risks at polling places. Investigations, audits, and studies from sources like the Heritage Foundation, Brennan Center, and NPR found voter fraud extremely rare, with only a handful of confirmed cases nationwide (e.g., under 50 in Heritage's database for 2020), none on a scale to affect outcomes.