By 2018, millennials will control $9 trillion in assets, according to a study by Deloitte. What they stand to inherit will grow by 4 times that amount, or $36 trillion, by 2061, according to Boston College’s Center on Wealth and Philanthropy. But don't expect this group to put that money to work in the markets, according to a new survey by Bankrate.com.

Instead, these young earners are keeping their money on the sidelines, choosing to save rather than invest their hard earned cash in stocks. Millennials, in fact, have the most conservative investment approach of any age group surveyed by Bankrate and that could be to their own detriment, given the market’s run up. 

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Bankrate.com released the results of a new survey about how secure Americans feel about their personal finances compared with 12 months ago. According to the results, Americans overall chose cash as their favorite long-term investment. In fact, 1 in 4 Americans prefer cash investments for money they will not need for at least 10 years. Stocks came in third with 19% of the vote.

Nearly 40% of 18-29 year-olds say cash is their preferred way to invest money they don’t need for at least 10 years -- despite the fact that the S&P 500 has gained 17% over the past year while the yield on cash investments is below 1%. Many Millennials are missing out on this market’s profits.

Bankrate.com’s senior financial analyst, Greg McBride, discussed the data with me and said there’s a real risk with this group because of the retirement savings burden they face.  He said, “[Millennials] don’t have the pensions that their parents had. Social Security has a very uncertain future. So they’ve got a bigger burden of accumulating wealth on their own for their retirement needs particularly in an era where life expectancies are getting longer, health care costs are going up. So hunkering down in safe haven investments for that long-term money, I think, runs a significant risk that you’re going to fall short."

Millennials are facing a number of unique factors other generations aren’t. These young adults are facing a tight job market and ballooning student loan debt burdens. They are increasingly living in multi-generational homes.  They also have a general distrust of the markets.

“They did have front row seat in terms of the impact of not just the financial crisis, but also the tech bust and what that did to their parents or friends and their portfolios," says McBride. "As a result, I think they’ve taken a pretty conservative investment stance just right out of the blocks."