BOSTON — Aaron Lifter has a clear goal in mind when it comes to investing. He keeps a little more than half of his portfolio in stocks. The rest is stashed in bonds to help smooth his returns.

With about 55 percent in stocks, he's hardly alone. It's a common balance for people approaching retirement who want to protect their savings, without investing so conservatively that they miss out on market rallies.

Yet Lifter isn't a 50-something baby boomer. He's 24, and could invest far more aggressively. The University of Miami law school student has decades to make up for losses he might suffer from the market's periodic declines. He built up his savings at an early age by spending two years helping to run the commercial real estate business that his late father founded.

His approach defies conventional wisdom that 20-somethings' should have as much as 90 percent of their investments in the stock market. That mix reflects the greater long-term returns that stocks historically have delivered compared with bonds, and is how target-date mutual funds try to strike an age-appropriate mix for investors expecting to retire around 2050.

But three years after the market's steep plunge, aversion to stocks is common for investors of every age. What's surprising is that the attitude has become ingrained in Generation Y, generally defined as people born from 1981 through 1995.

Lifter, for example, says he's relatively comfortable with risk compared with friends his age who keep as little as 10 percent of their investments in stocks.

A key reason is that they've seen losses as much as they've seen gains in the relatively short span that they've been investing, and watching their parents invest.

"We've had the tech bubble burst and the real estate bubble burst," Lifter says.

Their portfolios are frequently so conservative that they resemble those of investors decades older, although Gen Y has far longer to recover.

"Gen Y is investing more like their parents and grandparents, many of whom grew up in the shadow of the Great Depression," says Bill Finnegan, a marketing director with MFS Investment Management.

The mutual fund company conducted a recent survey to gauge the mood of investors. It found that Gen Y investors, ages 18 to 30, kept an average 30 percent of their portfolios in cash, such as money-market funds and bank accounts. That was nearly as much as the 33 percent they invested in U.S. stocks and stock funds. Among all age groups, cash averaged 26 percent.

The real eyebrow-raiser came from the 40 percent of Gen Y respondents who agreed with the statement, "I will never feel comfortable investing in the stock market."

Gen Y's cautious investing is a hot topic across the industry. They're "incredibly risk-averse," Sanjiv Mirchandani, president of Fidelity Investments subsidiary National Financial, said at a recent conference where he highlighted the trend. "They're putting away their savings in banks."

Many are cautious because they have little or no money to invest, and face bleak job prospects, even if they're well-educated. Nationwide employment among 16- to 29 year-olds last year was at the lowest level since World War II. Just 55 percent had jobs, down from 67 percent a decade earlier.

Those are some of the realities that young protesters in the ongoing Occupy Wall Street movement are calling attention to.

Finnegan, of MFS, believes Gen Y is becoming a generation of "recession babies" who are conservative about investing, much as people who came of age during the 1930s have been called "depression babies." The stock market volatility and economic hardships that group endured left them more averse to money risks than preceding generations.