In March 2009, in the midst of the financial crisis, the Dow hit a historic low, falling to levels not seen since 1996 — or 1966 if you take inflation into account. But in the three years since, the stock market has been on an "epic bull run," gaining more than 100 percent in value, says Roben Farzad at Bloomberg Businessweek. Yet lots of investors are still standing on the sidelines — trading volume on the New York Stock Exchange is at its lowest level since 1999. Why? Here, a guide to America's stock market skittishness:

Where are people investing their money?
In the past three years, American investors kept $710 billion in safe bonds, which produce meager returns, and only $260 billion in stocks, according to one study. Plenty of Americans aren't investing at all,  preferring to park their money in super-cautious savings accounts.

Why are investors avoiding stocks? 
The financial crisis spooked them. It was "the biggest and scariest plunge this generation of investors has ever seen," says Adam Shell at USA Today. And studies show that investors feel the impact of losses much more than the high that comes from profits. "The ability to scare the hell out of people is much greater than the ability to attract them to equities," Brian Barish, a leading investor, tells Bloomberg Businessweek.

Are investors right to be afraid?
It depends whom you ask. Some analysts say that companies are overvalued, and that their share prices are heading for a crash. Others argue that many stocks are still priced too cheaply compared to the profits businesses are posting. Either way, there are plenty of possible risks for the U.S. economy, including a spike in gas prices or an escalation of the European debt crisis. However, the "general consensus" is that the "bull market is alive and well," says Shell. If investors gain confidence and plunge back into stocks, it could "lead the market to new highs," says Bloomberg Businessweek.

What if the economy tanks again?
Despite its huge swings, the stock market has been fairly resilient over the years, says Morgan Housel at The Motley Fool. Investors who bought stocks in 2003 and held on to them gained an average of 5 percent a year — "not bad, considering we suffered the worst recession in 80 years, an oil spike, two wars, financial scandal, global terrorist attacks, record deficits, a downgrade of U.S. debt, and a near shutdown of the federal government."

Sources: Bloomberg Businessweek (2), The Motley FoolUSA Today