Growing up during an economic downturn scared 20-somethings into saving — but they’re also too skittish to do much with the money they’re socking away.

About 85% of millennials are putting away a portion of their paycheck, according to an online survey of 1,100 Americans conducted by digital ad agency Fractl on behalf of eBay Deals, published Friday. That’s a larger share than the roughly 80% of their parents, grandparents and slightly older brothers and sisters who save, the study found.

Young adults’ propensity to save is likely the result of coming of age during the Great Recession, when they watched the financial crisis clobber their parents’ retirement accounts and struggled themselves to find decent paying jobs even with college degrees, says Chibuzo Okoro, a campaign manager who authored the survey at Fractl, a digital marketing agency in Delray Beach, Fla.

“They’re worried,” she says. “They’re just anticipating being out of a job. You want to save as much as you can when you can.”

There’s just one problem, however: That skittish mentality has pushed 20-somethings to keep their money in the modern equivalent of under the mattress or in checking or savings accounts where it accrues little interest, instead of investing, says Stefanie O’Connell, the author of “The Broke and Beautiful Life,” a personal finance guide for millennials. Just 26% of Americans under 30 are investing in stocks, an April survey from personal finance site Bankrate found.

“This is where I think millennials trip themselves up,” says O’Connell. “They’re very conservative,” she adds, noting that young adults’ fear of the stock market means may are putting their savings in places where the money won’t grow enough to keep pace with inflation.

Instead, O’Connell suggests 20-somethings divide their savings between an emergency fund in their checking or savings account that’s equal to about three to six months-worth of expenses, an exchange-traded fund — also known as an ETF — to save for the medium-term and a retirement account. (ETFs are mutual funds that track stock market indexes.)

O’Connell recommends millennials try to put away about 20% of their income each month, though she noted that could be a difficult goal for many young adults who are still paying down their student loans. More than half of 20-somethings are saving just 10% or less of their paychecks, according to the eBay deals survey.

“A lot of investment of time and money is still going back into their careers and into being flexible and maintaining their options rather than long-term retirement savings or saving for a home down payment,” O’Connell says.

More than half of millennials have delayed a major life event such as buying a house, getting married or having kids, because of student loans, a recent Bankrate survey found. The weight of that debt and its ripple effects into other areas of their lives may be why one-third of young adults believe they’re living below their means, the largest share of any generation, according to the eBay Deals study.