Utah’s move to voting by mail has boosted turnout among millennials and other groups less likely to vote, according to new research released Monday, the day before the state’s 2018 primary election.

The study, conducted by Pantheon Analytics and commissioned by Washington Monthly magazine, showed a 5 to 7 percentage point turnout increase in the state’s 21 counties that vote by mail in 2016 compared to the eight counties that voted primarily on Election Day. And the by mail counties also saw the greatest increase in engagement among low-propensity voters.

“When it’s on the burden of the state or the county to get everyone a ballot, it makes it a lot easier for people to vote,” said Amelia Showalter, co-founder and CEO of Pantheon Analytics and the lead researcher on the study.

The numbers are based on turnout models, which look at an individual voter’s likelihood of casting a ballot.

Showalter, a former Obama campaign staffer, called the results “significant.” She said those in political organizations and campaigns who look to increase engagement in elections are often “thrilled” to see even one additional percentage point of turnout. An increase of the size found in the study, then, makes by-mail voting one of the most effective reform methods she’s seen.

In last year’s 3rd Congressional District race, the only two counties using traditional in-person voting saw far lower turnout than the other five. Salt Lake County saw nearly doubled its municipal elections turnout in 2017 compared to four years before.

Following these successes, an additional seven counties have implemented vote by mail since 2016. The only two that have stuck with traditional polling locations are Carbon and Emery. But Justin Lee, the director of elections in the Lieutenant Governor’s Office, said he expects that will change soon.

“Carbon and Emery are not very big counties, so the vast, vast majority of the population is in a primarily vote-by-mail county at this point,” he said. “We’ve gone from one county being vote-by-mail in 2012, to 10 counties in 2014, to 20 counties in 2016, and now up to 27 counties.”

Proponents of the vote-by-mail system say it’s more convenient, since residents may fill out their ballot whenever they like over several weeks and drop it in the mail. And it offers voters the chance to research candidates before making a decision. These elements may appeal to millennial voters, who have a track record of low voter turnout compared to other generations when they were the same age, according to the Pew Research Center.

“For younger votes who historically vote at lower rates, receiving a ballot in the mail may be a much-needed prompt for civic participation,” Pantheon Analytics concluded.

Although vote-by-mail may help boost low millennial turnout, it also comes with challenges, said Salt Lake County Clerk Sherrie Swensen, who noted difficulties maintaining updated addresses within the more mobile group of people.

In the 2016 election, three states conducted their elections by mail: Colorado, Oregon and Washington. But the research from Pantheon Analytics looked specifically to Utah because of the piecemeal way it implemented its vote-by-mail system, Showalter said, which offered a pseudo control group of traditional polling counties to compare vote-by-mail results to.

One area that demonstrates the divide between the two systems is in the SunCrest neighborhood — a housing development that straddles the border between Salt Lake County and Utah County. Neighbors on the northern side of the line were far more likely to vote in the 2016 election, the study found, with an increase in turnout as high as 12.5 percentage points due to vote-by-mail, the study concludes.

There were likely other factors at play, including the location of voting centers and slight demographic differences, but Showalter said those divergences wouldn’t account for that high of a gap.

Utah County Clerk Bryan Thompson agreed, noting “there’s no question” the by-mail ballots made it easier on Salt Lake County voters.

His county has since implemented its own vote-by-mail system and has seen increases in turnout since, Thompson said. Through it briefly planned to revert to a mostly in-person system this election to save money and finetune its election processes, the county backed off that plan in late March after outcry from local city officials.

In advance of Utah’s primary elections Tuesday, Thompson said ballot returns seemed “like a normal off-year election with lower turnout.” But in Salt Lake County, Swensen said returns have exceeded the 2016 primary, with a 33 percent turnout so far.

The postmark date for primary vote-by-mail ballots was Monday, but Utahns who haven’t mailed their ballots yet can drop them off at their county’s voting center until the polls close Tuesday at 8 p.m.