Millennials are buying mutual funds—perhaps because they have no other choice.

According to recent research from the Investment Company Institute (ICI), millennials born between 1981 and 1998 are more likely than older generations to hold mutual funds only in their retirement plans.

Among millennial households in ICI’s survey, 45 percent held mutual funds only through their retirement plans. Among baby boomers, headed by individuals born between 1946 and 1964, 34 percent held mutual funds exclusively in workplace plans.

Having come of age at a time when ETFs dominate the headlines, millennials aren’t necessarily a natural fit with mutual funds, but thanks to the lack of choice within workplace retirement plans, young adults are buying them nonetheless.

Among households that purchased their first mutual fund in 2010 or later, 71 percent made the purchase through a workplace retirement plan, compared with 56 of the households who made their first purchase before 1990.

Overall, three-quarters of U.S. households who own mutual funds hold them within tax-deferred accounts like employer-sponsored retirement plans or IRAs.

Almost all mutual fund-holding households, 92 percent, said that retirement saving was one of their financial goals.

Most mutual fund shareholders have moderate or lower household incomes and were in their peak earning years, said the ICI. A little more than half of mutual fund-owning households had incomes less than $100,000, and individuals between the ages of 35 and 64 headed almost two-thirds of these households, or 63 percent.

Millennials are also buying their first mutual funds at younger ages than previous generations. The median age at which households headed by adult millennials first purchased their mutual funds was 23 years old. The average age of first investment becomes older in older generations. For example, members of Generation X, born between 1965 and 1980, were on average 27 years old when they first purchased mutual funds, while baby boomers reported being in their 30s before making their first purchase.

More than 44 percent of U.S. households owned shares of U.S.-registered investment companies, in mutual fund, ETF, closed-end fund or unit investment trust form, with mutual funds being the most common type of investment.