Universal Voting-By-Mail Increases Voter Turnout for Both Major Parties

In recent years, voting by mail has been the subject of numerous lawsuits and attacks that often claim the process gives Democratic nominees and legislative bills an edge. But a recent Caltech study shows that sending ballots to all registered voters boosts turnout for both major parties. Co-authors R. Michael Alvarez, Caltech's Flintridge Foundation Professor of Political and Computational Social Science and co-director of the Caltech/MIT Voting Technology Project, and Yimeng Li (PhD '22), a former graduate student in Alvarez's group, outlined their findings in a paper published in the Journal of Politics.

"We found a three to four percentage-point increase in turnout due to the policy," says Li, who is now a postdoctoral fellow at Florida State University. "The impact was larger for partisan voters compared to nonparty affiliate voters but among partisan voters, we saw no evidence of Democrats getting an advantage, which has been a common narrative."

While absentee voting by mail has been an opt-in option—mostly for the armed forces—for more than two centuries, COVID-19 spurred a shift toward universal and "no-excuse" vote-by-mail policies in many states. But even prior to the pandemic, California was expanding its mail-in voting policies to make it easier for people to vote by mail. Aided by a unique example of policies in flux, Alvarez and Li investigated how voting turnout was affected when all registered voters received a mail-in ballot automatically.

In early 2020, Li and Alvarez were already working with California's secretary of state when they happened upon an interesting provision that was included in California's 2018 Voter's Choice Act (VCA). The provision required that Los Angeles County send mail-in ballots to all registered voters living in parts of the county represented by congressional or state legislative districts that overlapped with neighboring Orange County, where universal voting had been rolled out as part of the VCA.

The VCA allows counties to be more flexible in how they conduct elections by exercising options such as mailing every voter a ballot and expanding in-person early voting. By the start of 2020, select counties, including Orange County, had already rolled out a full implementation of the VCA. To keep the consistency of this practice within the congressional districts and state legislative districts, Los Angeles County was also required to mail ballots to voters in the county who live in districts that also span Orange County, while the majority of voters in Los Angeles did not automatically receive a ballot by mail.

"It was just one of these arcane things that happens with elections and public policy in general, but it led to a really fascinating experimental design that we were able to exploit using a lot of the data that we had available to us at the time," says Alvarez, who is also co-director of the Caltech Linde Center for Science, Society, and Policy (LCSSP), which aims to bridge the science happening at Caltech with today's most pressing policy conversations, including those about democracy and elections.

By comparing voters in Los Angeles County who received a mail-in ballot automatically due to the new policy for the March 2020 primary election with those who did not, the researchers were able to see a clear uptick in voter turnout among those who automatically received a ballot by mail. They were also able to take advantage of data from a short window during which differences in mail-in ballot availability existed in a very geographically close population, as universal mail-in ballots were adopted statewide in November 2020.

"It's a particularly powerful design because some of the methods that we use examine the differences for voters who are effectively in neighborhoods with similar demographics; all that divides them is just this arbitrary congressional or state legislative district line," explains Alvarez, who notes that previous work by other researchers often compared voters from different counties or states. "We were able to establish probably the tightest treatment and control that has been seen in this literature because they're in the same neighborhood."

Alvarez and Li found that when all factors other than mail-in ballot status remained unchanged among voters, universal voting by mail delivered improvements in voter participation that did not favor one political party over another.

"I think conventional wisdom these days is that these kinds of convenience voter reforms have a partisan advantage, but we find that's not true," says Alvarez, who has a long history in electoral politics and voter behavior research. "This is something that we easily can do that does help boost participation in democracy and doesn't seem to have any clear partisan ramifications."

According to Li, a 3 percent increase in turnout is substantial, especially compared to other factors that political scientists have studied; TV advertising, for example, has shown to have almost zero impact on turnout.

"Our study gives people actual evidence of the impact rather just a perception that might be wrong," Li says. "It also provides input for policymakers so they can make informed and sound policy decisions."

Li and Alvarez continue to work together to investigate voting behavior related to changes in ballots. They are currently examining the impact of a recent change in Los Angeles County voting laws that places local races at the top of the ballot, before federal races, with the aim of boosting participation in local issues.

Alvarez also recently received a grant from the MIT Election Data and Science Lab to support ongoing work with Orange County Registrar of Voters officials that is evaluating the accuracy and quality of voter registration data.

"It's wonderful to have opportunities to engage directly with election officials to make sure that the research we're doing is answering questions that are important to policymakers and helping provide science and data to inform what kinds of changes we implement in California and also nationally," Alvarez says. "This research helps us better understand our democracy."

The Journal of Politics paper is titled "Universal Mail Ballot Delivery Boosts Turnout: The Causal Effects of Sending Mail Ballots to All Registered Voters." It was published online in March 2026 and will appear in the July 2026 print version of the journal.