Just over three years ago - on March 9, 2009 specifically - the Dow Jones Industrial Average closed at 6,547.   It would mark the beginning of the greatest investment opportunity in a generation.

Don't recall it that way? Not many people do. Up to that point, stocks had been in an uninterrupted free fall. The Dow began 2009 by posting declines on 29 of the first 45 trading days for a total decline of 27% in just 10 weeks. And this was on top of the 25% decline the Dow experienced in the last three months of 2008.

Fueled by the constant media barrage about how bad things were and how much worse they could get, folks weren't much thinking about putting new money into the market. To the

contrary, they were in full panic mode, pulling out of stocks and pouring into the "safe havens" of cash and bonds, willing to lock in their losses for fear of even greater declines perceived to be ahead.

And yet...

From that point forward stocks would experience the greatest three-year bull market of the past century. An investor who put $100,000 into the S&P 500 would have seen his investment more than double in just 36 months to $212,931 (113%). If he had put it in the Russell 2000 small stock index, his investment

would have been worth $239,125 (139%). And in the Wilshire REIT index: a whopping $337,834 (238%).  

Unfortunately, this has not been the experience of the average investor. In an article titled "Main Street's $100 Billion-Dollar Blunder," (smartmoney.com, February 28, 2012), columnist Brent Arends did some back-of-the-envelope calculations about what investors had cost themselves since the financial crisis of 2008 purely through bad behavior:

In October 2008, after Lehman, investors panicked and withdrew about $45 billion from their U.S. stock funds.  That trade alone has cost them $25 billion in investment profits since, according to MSCI data: On average, the shares they sold for $45 billion would be worth about $70 billion (including dividends) now. In February and March of 2009, as the market slumped to its record lows, mutual fund investors sold another $29 billion worth of U.S. stock funds.

That cost them another $26 billion in lost profits.  In total, by my math Main Street investors have missed out on a staggering $106 billion in investment profits over the past five years by selling stocks at the wrong time.

It's not as if investors made their hasty decisions in a vacuum. Indeed, to listen to the media during the turmoil, the only logical conclusion one could have drawn was that it was folly to sit around and watch your stock portfolio continue to dwindle.

(And it should be noted, with no small irony, that Mr. Arends was among the worst offenders at prodding his readers to make short-term market timing moves during the financial crisis.)

The bottom line is that it takes tremendous resolve to keep the faith when virtually all others around you are losing theirs, but

that is what separates the very few successful investors from the multitudes who fail.  

In order to earn the long-term returns that stocks, and only stocks, have historically delivered, you have to be willing to experience the inevitable bouts of volatility along the way.

In fact, it is a perverse reality about stock investing that the times of greatest turmoil in the market are also the times of greatest opportunity.  

This is why your own behavior, not the machinations of the market, is far and away the greatest determinant of your long-term success in saving for retirement.