If you invest in stocks for the long term, you must own small-cap stocks.

There, I said it. But come to think of it, that's not personal investment advice at all. That's Wall Street's worst-kept secret:

Over the long haul, smaller-company stocks outperform their mid- and large-cap peers, so smart investors own them. Period.

Only what you need
I'll back that up with some hard data, but first let's talk about you. You're serious about this stuff; otherwise, you wouldn't be reading. You also want an edge -- a secret -- that will help you do better than the average investing Joe. We all do.

So why make this harder than it needs to be? Investors who make the most of their savings over the long term hold diversified portfolios of common stocks. At least they have since Ibbotson Associates started keeping tabs back in 1926. Stock investors who make even more buy small caps, also according to Ibbotson.

Sticking with our "simple" theme, here's how I see it. You can buy a small-cap fund that keeps its costs in check, though the good ones are as hard to get into as decent preschools. You can buy one of a growing number of great small-cap exchange-traded funds (ETFs). Or you can seek out -- with or without the help of a trusted advisor -- the very best smaller companies and build a small-cap portfolio of your own.

You're a Fool...
... And so am I. So you'd think we'd favor the do-it-yourself approach. Well, sort of. I've been around, and I own my share of small caps. But I also have the occasional cup of joe with Motley Fool co-founder Tom Gardner, who works 24/7 digging up well-run small companies for subscribers to his Hidden Gems advisory service -- and he never shuts up about it.

So I think about this a lot. And you know what? For better or for worse, Tom's more disciplined, strict value approach has resulted in a portfolio of small-cap gems I wouldn't have built on my own. One that -- gun to my head, and assuming I'm in it for the long term -- I would swap straight up for mine. There, I said it again.

For all that, Tom and I look for many of the same things in a great small company. It's what many of the all-time great value investors have always looked for. For more detail on this, check out my mild rant, How to Beat a Choppy Market. But here's the short list for now:

  1. Solid management with significant stakes;
  2. Great, sustainable businesses;
  3. Dominant positions in niche markets;
  4. Sterling balance sheets; and
  5. Strong free cash flow.

Good work if you can get it
I know what you're thinking: Who in his right mind wouldn't want a portfolio filled with stocks like that? And you're right. That's precisely the problem with trying to beat the market pros with well-known, large-cap stocks -- if they're really all that, they're going to cost you. How much? An efficient market theorist would say just enough. I'd argue too much.

So what are you going to do? Take a flyer on some fly-by-night outfit? Good point. But notice I specifically said well-known stocks, not well-known companies. Corning (NASDAQ:GLW), JDS Uniphase (NASDAQ:JDSU) -- those are well-known stocks (you even remember what those guys do?). McDonald's (NYSE:MCD)? Now, that's a well-known company and stock.

You want something in between. Fortunately, there are hundreds of established, profitable companies -- some you know, some you don't -- with lesser-known stocks. Some even dominate their markets. Peter Lynch was a master at finding these guys, earning his Fidelity Magellan fundholders nearly 30% per year. With a little work, you can do it too.

Yes, you can
Here's how I know. I stumbled myself upon Harley Davidson (NYSE:HDI) the stock after having seen (and heard!) the bikes for years. Analysts were debating Delta (NYSE:DAL) and pals, but Southwest (NYSE:LUV) got me home on time. Or think Taser (NASDAQ:TASR) just a short year back -- heck, we'd all seen Cops, right?

There are exceptions, of course, but the fact is it can take a long time for a stock to cross the big boys' radar, even if they swear by the company's products themselves. And when it does, it usually hogs the radar screen long after it should have blipped off. That gives us a great advantage as individual investors. We can find these stocks and buy them before the big money drives them to the stratosphere.

Take your time
Back in September 2003, I suggested you get the ball rolling with a hard look at a pair of small-cap exchange-traded funds (ETFs). I'd bought the iShares S&P 600 Growth Index (IJT) myself at about $65 earlier in the year and was thrilled with my returns. I pledged to buy the sister fund iShares S&P 600 Value Index (IJS) next -- a promise thankfully kept.

In just the 15 months since, the growth fund is up another 30%. The value fund has fared even better -- gaining 37%. I don't bring this up now to brag, but to make two points: First, all who said that the small-cap rally was over back in September 2003 (and they were many) were flat-out wrong. (My hunch is there are even more saying it now -- and that they're still wrong.)

More importantly, these funds, which trade like stocks, offer you quick and dirty small-cap exposure without the risk -- perceived or otherwise -- of taking the plunge on the stocks of individual companies. The strategy of holding the funds, then shifting into Tom Gardner's Hidden Gems after digging at your leisure, strikes me as sound. As does simply holding the funds as a means of diversifying one's small-cap exposure.

Buy the numbers
Earlier, I promised you numbers. Ibbotson Associates is the undisputed king of market data. According to Ibbotson, since 1926, small-cap stocks have delivered annual returns of 12.4%. That's compared to 10.7% for large caps. Put another way, $5,000 invested in small-cap stocks and left to sit would grow into about $52,000 over the course of 20 years. Imagine if you'd been adding to your position. Nice.

And that's overall. In periods when small caps do outperform, they seriously outperform, and they tend to do so for periods lasting -- depending on whom you ask -- from five to seven to 10 years or more. Frankly, I don't think the small-cap run is over. Either way, it's axiomatic: Today's giants sprouted from well-run small companies, taking smart investors along for a swell ride. So will tomorrow's.

What to do now
I'll follow up on many of these themes in future columns. Like I said, I'm a small-cap investor, and I think about this stuff a lot. To learn more about Tom Gardner's approach to beating the market with small-cap value, Tom's offering a free trial to his Motley Fool Hidden Gems advisory service. You can take it up directly with him and sneak a peek at all his back issues. Simply click here to check it out.

Meanwhile, I promised to keep you posted on Hidden Gems' performance. As of January 6, 2005, the recommendations are up, on average, 38%. That's compared to 9.1% if you'd invested in the S&P 500 for the same period. For context, that's 38 picks over as many months. As always, you can view the entire scorecard with your free trial.

Got some gems of your own? Shoot me a note and let me know what you've got.

Fool writer Paul Elliott owns none of the stocks mentioned. The Motley Fool has a full disclosure policy.