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It makes sense for little kids to keep their money in a piggy bank—but a staggering number of adults these days are still basically managing their money the same way.

A new survey from Bankrate.com finds that 23% of American adults think cash, in the form of savings accounts or CDs, is the best place to keep a long-term investment.

It’s not. Sticking your money into the grown-up equivalent of a piggy bank not only guarantees that you lose out on whatever you could earn by investing it in the stock market, but also that inflation will erode the value of what you saved, meaning you’ll have less spending power when you do take that money out.

Women tended toward this line of thinking more than men: Almost one in three women felt cash was the safest place for their money, compared with 16% of men.

There is undoubtedly some fear that is evident across the board,” said Greg McBride, Bankrate’s chief financial analyst.

Financial fear, along with perhaps a lack of knowledge about investing, is most prevalent among millennials. A whopping 38% of respondents under the age of 30—those who have the most to gain from a long stretch in the stock market and the most to lose in terms of the threat posed by inflation over the long haul—said they preferred to keep their money in cash.

Although the stock market carries risk, these Gen Y members are opening themselves up to potentially unprecedented financial instability in their own retirement years.

“Millennials will have the biggest retirement savings burden in history due to longer life spans, higher health care costs, fewer pensions, and the uncertainty of Social Security,” McBride said. “Hunkering down in safe haven investments will leave you light years short of where you need to be for retirement,” he cautioned.

Across all age groups, only 16% of respondents said the stock market was the best place to keep money over the long term — the same percentage that said gold or other precious metals were the best option.

While precious metals could play a small role in a diverse portfolio, people who rely on it as a way to preserve their assets are making a big mistake, McBride said.

“They can act as a hedge against depreciating currencies or a safe haven in an economic calamity, but they generate no dividends or interest income,” he said of investments in gold and the like. Just to stay on top of inflation and break even in the future, other people must be willing to pay more than you invested, a situation McBride called the “greater fool theory.”

The largest number of people surveyed (25%) believe that real estate is the best place to stash money not needed for 10 years or more. Real estate isn’t perfect, McBride said—it barely keeps up with inflation and is very illiquid—but it’s the vehicle of choice for one in four Americans.

On the upside, he noted, “It is appropriate for long-term investing.”