Millennial investors, those 34 years and younger, can’t forget the market meltdowns of the last 15 years and are badly investing.

Those are some of the conclusions of a just-released MFS Investing Sentiment Insights Survey. “The data confirm we have a lost generation of investors,” said William Finnegan, senior managing director and head of global retail marketing for MFS.

The survey showed that millennials typically hold large amounts of cash in their portfolios, 25.8 percent, and only 30.5 percent in stocks. The millennials also misunderstand basic investing concepts. They confuse active and passive investing, often mixing the two. And they consider long-term investing to be somewhere between five and seven years, according to the survey, which polled advisors and investors.

Some 46 percent of millennials say they never feel comfortable in the stock market. The pessimism of young investors has worsened. Two years ago, the number was 40 percent. Finnegan says the survey proves that advisors have “lots of work” to do with young clients, educating them about the need for stocks to achieve long-term goals.

“The impact of 2008’s Great Recession has had a deep-seated secular impact on millennial investors,” Finnegan added. Ironically, he noted that the grandparents of millennials are more comfortable about investing in the stock market.

“The grandparents are more aggressive investors,” he added. But their grandchildren, another MFS official noted, are behaving the way many people did who lived through the Great Depression of the 1930s.

“I think the background on this is very similar,” adds Doug Orton, vice president of business development for MFS Investment Management. “If you picture the Depression babies, the reason they were so impacted by it is that they were young adults during the Great Depression, in the 16-to-24-years age range. If you think about the millennials, they were 18 or 19 during the tech wreck. Then they were in their 20s in the last downturn. These could have a very similar and long-lasting effect on their attitudes toward investing.”

Orton said he was surprised by the investment pessimism of millennials. That’s because the stock market returns since the meltdown of 2008 have been good. So why are young people putting their money in cash and shunning the bull market of the last five years?

“I think it is because of the economy. Their pessimism about the economy and whether they will have a job is affecting how they invest,” according to Orton. How should advisors approach the problem? Advisors should emphasize the long term returns of equities and steer away from discussions about business cycles, Orton said. The survey also showed a disparity between advisors and investors in how they view the economy. Some 79 percent of advisors were upbeat on the economy over the next five years while only 44 percent of investors are optimistic.

MFS officials also said that advisors must educate young investors about the difference between passive and active styles.