(Bloomberg) -- A couple of years ago, the University of Arizonaadded an option to its retirement plan. Instead of contributingpretax money, which is the traditional option for 401(k), 403(b), and other workplaceplans, workers could contribute aftertax money to aRoth account.

Employees weren’t sure what to do. Among them was ScottCederburg—and he's a professor of finance.

Researching the question didn’t help much, despite hisexpertise. “There weren’t really good, solid answers,” saidCederburg, who teaches at the university’s Eller College ofManagement.

Ever since Congress created Roth accounts , savers have facedthis quandary: Roth or traditional? The experts’advice is often vague, noncommittal, or downright contradictory. Inother words, “it depends.”

It depends, for example, on what tax rate you’re paying now,compared to what tax bracket you’ll be in when youretire. Traditional 401(k)s, along with traditional individualretirement accounts, lower your taxes now, but you end paying atthe other end, when you take the money out in retirement. With aRoth account, you pay taxes now, but all withdrawals later,including investment gains, are tax-free.

It’s pretty clear, then, that Roth accounts should work well foryoung, low-paid workers, who are probably paying the lowest taxrates of their careers. And a recent study suggested thatworkers may end up unintentionally contributing more to Roth401(k) accounts than to traditional 401(k)s, albeitunintentionally.

The benefits of a Roth are less obvious for older or wealthierworkers, who are paying high tax rates now and don’t know whattheir taxes will look like in 20 or 30 years. How do they balancelowering their tax rate now, using a traditional account, with theadvantages of a Roth’s tax-free pool of money in retirement?

Cederburg decided to find out. A new study, which he conducted with two other finance professors—David C. Brown of theUniversity of Arizona and Michael O’Doherty of the University ofMissouri—used software to simulate the optimal saving strategiesfor a million retirement scenarios over the next 30years.

Their conclusion: Most investors, including wealthy ones, end upbetter off with a mix of assets in traditional and Rothaccounts.

Each kind of account has advantages, the study found.Traditional 401(k)s and IRAs can be used strategically to loweryour current tax rate, which is especially valuable when atraditional contribution bumps you down to a lower taxbracket. Roth accounts insure against the chance that tax rateswill spike as baby boomers retire, health-care costs rise, or thenation faces a more serious fiscal crisis.

“It’s tough to say what the world will look like in 30 years,”Cederburg said. “With a Roth account, you can lock in current taxrates.”

The study modeled the future based on past investment returnsand tax rates, which were anything but stable. Since 1913, themarginal tax rate has changed 39 times for a single person with aninflation-adjusted income of $100,000, varying from 1 percent to 43percent. The wealthiest Americans have paid an even wider range ofrates. For 15 years in the mid-20th century, the top tax bracketwas above 90 percent.

While well-paid workers are often told to skip Roth 401(k)sentirely, the new paper could change that advice. "This suggeststhat Roth accounts should be given a more serious look by those inhigh tax brackets,” said Daniel Ostrov, a Santa Clara Universitymath professor who has written about (PDF) Roth andtraditional retirement strategies and who praised the Cederburgstudy as “quite good.”

Though the study tried to quantify optimal strategies, findingsignificant benefits in mixing Roth and traditional accounts foralmost all savers, its simplified model doesn't offer a recipe.Cederburg and his colleagues couldn’t take into account certaindistinct advantages of each option. Assets in a Roth are easier towithdraw before retirement and don’t require mandatory withdrawalswhen you get older. Traditional accounts can sometimes be convertedto Roth IRAs in a way that avoids taxes. And there's always thesmall chance that Congress could change the rules for Roth accountsand make them less valuable.

For his part, Cederburg, 34 years old and saving for aretirement that won’t start until 2050, decided to split hiscontributions 50/50 between Roth and traditional options. Manyemployees don’t have the option. A Vanguard Group study last monthof its own 401(k) plans found that 40 percent of employers don’toffer Roth accounts.

“What is clear from the paper," Ostrov said, "is that everycompany should give their employees a Roth option to save forretirement.”

Copyright 2018 Bloomberg. All rightsreserved. This material may not be published, broadcast, rewritten,or redistributed.