What's keeping Millennials away from the stock market?

Among the biggest reasons, says a new survey, is that they can't afford to -- an obstacle that prevents older generations from investing as well.

A majority of Millennials, the generation of people who in 2015 were between the ages of 18 and 34, do not invest in the stock market, which includes buying individual company stocks, bundles of stocks through mutual funds or exchange traded funds, and contributing to retirement accounts such as 401(k)s.

According to phone interviews with 1,000 nationally representative adults, results from which were released this week by investment information website Bankrate.com, just 1 in 3 Millennials have money in the stock market. The main reason so few invest is that they can't afford to.

“Money is a big problem,” Jill Cornfield, Bankrate’s retirement analyst, tells The Christian Science Monitor. And not just for Millennials, she says.

Based on Bankrate’s survey results, nearly half of all Americans, 48 percent, say money is their main barrier to investing; a quarter of respondents say they keep out of the market because they don’t understand stocks. Then there are the 11 percent of people who say stocks are too risky, and 7 percent who say they don’t trust stock brokers or advisors.

“Not investing is an American problem for all age groups,” says Ms. Cornfield.

Collectively, 4 in 10 Americans, 43 percent, own stocks, reports Bankrate. This includes 51 percent of people who are 36 to 51 years old (Generation X), and 48 percent of Baby Boomers, who are aged 52 to 70.

Among older Millennials, those between 26 and 35 today, the investment picture is a little brighter, though not ideal, according to the survey. In that age group, 44 percent of people invest, compared to less than a quarter of 18- to 25-year-olds.

“I think the reasons for that, is that they [older Millennials] have more money, but they have more demands on their money: they want to start families, buy homes, furnish them, travel,” Cornfield says. "Younger people are resigned to not having money."

Despite that low participation in the stock market, Bankrate’s previous research has found that Millennials actually save more than previous generations, for both emergencies and retirement.

"Much of this is attributable to the financial crisis and Great Recession coming during the financially formative years for many Millennials,” Greg McBride, Bankrate’s chief financial analyst, said in a statement in March.

An earlier Bankrate survey found that 62 percent of Millennials will save more than 5 percent of their income this year, an increase from 42 percent in 2015. A third of Millennials are saving more than 10 percent of their income, which is up from 22 percent last year.

According to Cornfield, all Americans should aim to stash away 10 to 15 percent of their income towards retirement savings.