Millennials with very few assets and baby boomers with the bulk of America’s assets are both popularizing the use of exchange-traded funds, according to a new research report from mutual funds giant Vanguard.

A survey by Vanguard of five million of its retail investors found that the number of folks using ETFs has more than doubled since 2013, reaching 11 percent of Vanguard investor households in 2018. At the same time, mutual funds remain the dominant holding, particularly for Vanguard’s older investors with years of capital gains; 83 percent of Vanguard retail households still held mutual funds at the end of 2018, the report found.

Exchange-traded funds are baskets of stocks or other assets that come with lower cost for investors. Typically, it’s more expensive to buy all the individual stocks held in an ETF portfolio. Some brokerages even offer no-commission trading on certain low-cost ETFs.

Assets in U.S.-registered investment companies rose in 2017 to a record level at year-end of $22.5 trillion, with mutual funds and ETFs accounting for about $22.1 trillion according to Investment Company Institute. At the end of that year, there were 1,569 index-based ETFs with $3.3 trillion in total net assets.

Many new investors in ETFs are millennials, Vanguard found, in particular young people with lower account balances and using ETFs as mainstays in their portfolio holdings.

In 2018, 17 percent of new investors at Vanguard purchased ETFs, compared with 6 percent in 2013, according to the study, co-authored by Jean Young and Thomas DeLuca.

“ETFs are cheap and there’s no account minimum to buy,” said Young.

It is not surprising that boomers tend to hold more money in their accounts at Vanguard. With a median age of 58 and a median account balance of $344,000, these older investors are often “diversifiers,” said Young, senior research associate at the Vanguard Center for Investor Research, in an interview.

“ETF diversifiers are older, affluent, and long-tenured Vanguard households,” she said. These retail investors are more likely to try an ETF as a way to diversify a long-standing portfolio.

ETFs represent 7 percent of these longtime clients’ portfolios and part of a more complex investment landscape, with the typical ETF diversifier having three accounts and 12 distinct holdings.

More established affluent investors tested the waters — using ETFs in select ways.

“It was surprising initially, but a longtime investor with Vanguard may choose to fill certain parts of a portfolio with an ETF. They wouldn’t redo a whole portfolio of mutual funds, especially with embedded capital gains” on which they would ultimately owe taxes, Young said.

At the other extreme are what Vanguard dubbed ETF “enthusiasts.”

“Sometimes these are younger investors, with a median age of 36, and are also relatively new to Vanguard, with a median account tenure of three years,” Young said.

ETFs represent 97 percent of “enthusiast” portfolio assets, almost overwhelmingly in stocks.

Enthusiasts are lower-wealth investors—with about 4 percent of the median wealth of ETF diversifiers-- with simpler portfolios, typically holding a handful of investments in a single account.

Finally, there’s the oldest set of investors, over the age of 60, which Vanguard called the “silent generation” ETF enthusiasts. They’ve also have embraced these vehicles, investing equity allocations close to 90 percent, similar to millennials. The typical silent generation household allocates 63 percent to equities, the study found.

Determining whether an investor will try an ETF depends on a few factors, including age, account balance and whether the person is new to Vanguard, the study found.

One other factor Young said Vanguard may study further: gender breakdown of investors using ETFs.

“Clients have told us they’re really like to see that,” she said.

Vanguard has cut fees on a slew of ETFs over the last year. Some new expense ratios are 0.01 percent to 0.02 percent (or 1 to 2 basis points) below the expense ratios of the respective funds’ Admiral shares.

The ETFs with lower expense ratios include Vanguard’s FTSE Developed Markets (VEA); Growth (VUG), Large-Cap (VV) Mid-Cap (VO); S&P 500 (VOO); Total Bond Market (BND); Total Stock Market (VTI); and Vanguard Value (VTV).