Today, everywhere I turn in the investment world, someone is asking about the merits of long-term investing. Does it work? Did it ever? Looking over the wreckage of one-time blue chips AIG (NYSE:AIG), Fannie Mae (NYSE:FNM), General Motors, et al, these are natural questions.

For every Danaher (NYSE:DHR), up around 3,000% over the past 20 years, there are plenty of disappointments -- such as Eastman Kodak (NYSE:EK), down nearly 90% over that same time frame.

So which is it? If you buy and hold for 25 years, are you a champ or a chump? If you stick with a stock for five years, are you a star or a sucker?

What do you think? (Drop a comment below!)

Here's what I think.
It makes sense that this debate would rage during the most explosive year in the history of stocks, with the S&P 500 racing to 1,400, buckling to 670, and now settled 'twixt the two. In 2008, on 18 separate days, the index moved more than five percentage points. Damnation!

It took nearly 60 years to rack up the same number of highly volatile days. When decades of tiny waves give way to a single tsunami, you can no longer blindly accept the merits of body surfing. The same goes with long-term investing; we can't help but debate it after a year of calamity.

Now, if you think I'm chalking this up to mere shortsightedness -- a one-year phenomenon -- hold on there. The Motley Fool has been associated with long-term, buy-and-hold investing since our founding in 1993.

And as co-founder and CEO, I'm writing to say that, for many people, long-term investing simply doesn't work. You read that right. Many investors today are wrong to anchor their stocks to an average holding period of three to five years or more.

But now, are you ready to be confused?
All that said, the evidence still shows that multiyear or multidecade holding periods will generate the highest real rates of return, after taxes and the frictional costs of trading.

Lifelong stock investing is the most broadly available way, across the world, to become a millionaire. It's just that most people don't have the time frame, the temperament, or the training to invest this way. Nor do they tack on new capital to their portfolio at lower prices.

Master investor Shelby Davis did. By repeatedly buying and holding stocks forever -- Chubb (NYSE:CB), Aon (NYSE:AOC), and Torchmark (NYSE:TMK) among them -- he turned $50,000 into $900 million over a half-century of investing. Against that record, you have reams of statistical data showing that when investors trade their accounts actively, the odds of their losing to the market rise dramatically. Worse still, the likely outcome of their hyperactive trading is that they lose money outright (see Terrance Odean and Brad Barber, "Trading Is Hazardous to Your Wealth").

So what's the average investor to do -- caught between the rock (of not having the time or training to invest prudently for the long haul) and a hard place (of facing insurmountable odds against successful active trading)?

For me, it starts with knowing thyself. You must know the following (the four Ts):

1. Temperament. Can you stomach a 50% loss in the value of your investment portfolio over a two- to three-year period?

2. Time frame. Can you handle 10 years of zero returns from your investments?

3. Training. Are you capable of investing in public companies, diversifying internationally, and understanding what you own?

4. Tacking on. Are you inclined to add new money along the way, particularly as prices fall?

If you can't chant a resounding yes to all four of these, you shouldn't embrace buy-and-hold investing for the long term.

The facts bear out that most people don't demonstrate all four in their investment approach. They can't endure volatility leading to flat returns over any decade. They don't truly understand the businesses in which they've invested or the money managers with whom they've invested. And they are disinclined to add money during bear markets.

They should not buy and hold for the long haul -- period.

It is for these reasons that we launched our Motley Fool Pro service, smack in the middle of the mayhem last fall. Pro has almost never been in the red; it is up 10% today; and it is beating the market by 5.8 percentage points. But how?

The service features buying stocks long, selling stocks short, using exchange-traded funds (ETFs), and deploying options as a conservative hedge. The aim is to:

1. Dramatically reduce volatility.

2. Avoid down years.

3. Protect investors against ignorance, via an open community.

4. Eliminate the need to add new capital each year.

It aims to neutralize the need for a steely temperament, a multidecade time frame, the training to understand each investment, and the necessity of tacking on new capital each month, quarter, or year. This is precisely what must be done by the many investors for whom long-term, buy-and-hold investing simply is not suitable.

The debate
And so as I see it, the debate about long-term investing is actually a poorly framed one. Long-term investing simply doesn't work for many, many investors today. But that doesn't mean it's dead.

For those for whom it does, I expect they'll be enjoying financial freedom measured in quarter-centuries. For those for whom it is not suitable, they must learn diversification and hedging strategies.

We're having a big debate around this question here -- with Fools voicing strong opinions at both ends of the spectrum. Over the coming weeks, we'll be interviewing experts and voicing our own motley opinions. But we also want to know what you think.

So, what do you think? I look forward to reading your opinions and mixing it up with you in the comments area below, Fools!

Tom Gardner is CEO and co-founder of The Motley Fool. Tom does not own shares of any companies mentioned. Read about the Fool's disclosure policy here.