A long, long, time ago in Fooldom, my then-colleague, Randy Befumo, wrote "Rats." It's about a famous series of experiments by behavioral psychologist B.F. Skinner, in which he trained rats to push a lever in order to be rewarded with a food pellet.

Some of Skinner's rats were rewarded each time they pressed the lever; others were rewarded after every second or third press. Some were rewarded randomly. Once the rats got the hang of things, the pellets stopped.

How long did it take them to figure that out? Well, the ones that had received a pellet with every press learned first, followed by those that had to push twice, and then the three-press rats. The ones that took the longest to learn were those that had been rewarded randomly. For a long time, they kept pressing and pressing and pressing.

The rat connection
I can think of a few times I've behaved like a Skinner rat.

For example, almost every time I enter an Ocean State Job Lot store, I look for a pair of jeans similar to a wonderful pair I bought there four years ago. I haven't found anything like them since, and the store probably just doesn't carry them anymore. But I keep looking. (Hey, you never know, right?) See what's going on here? I got a pellet four years ago, and I keep trying to get another.

I'm not a regular lottery ticket buyer, but I do have what-if-I-won daydreams. This is really pointless, since anyone's odds of winning a multimillion-dollar jackpot are astronomically low. But I'm focusing on some other people's pellets. Yes, a very, very small number of people have won. They received a pellet after pressing the lever (usually many, many times). This can make us think we have a decent shot at getting a pellet, too. Think again!

Think also of why some people stay married when they might be happier divorcing and finding someone new. They got some pellets long ago, and they keep pressing that lever, thinking they might get some again.

If you don't stop and ask yourself why you're doing what you're doing now and then, you might end up wasting a lot of time or money, waiting for pellets that will never come.

Investing like rats
Meanwhile, there's the world of investing -- which might also be called Pelletville. It's really all about lever pushes and pellets, isn't it? You invest in Disney (NYSE: DIS) in January 2003 and by the end of the year, you're up ... 44%! You get used to that, and at least subconsciously, you sort of expect lots of stellar years.

In 2004, you earn 20%, and in 2005, you lose almost 13%. You're smarter than a rat, though, so you start doubting the possibility of a 44% return. But in 2006, you earn ... 44%! I suspect that many investors would now keep hanging on, waiting for very unlikely 44% returns, even though the 10-year average annual return for the stock is actually negative.

Another pellet-driven mistake that can cost you is investing on the fly, on a whim, on a hot stock tip, on a penny stock you read about in a spam email. Every once in a blue moon, these "investments" pay off. But that's luck, not smart investing. And whenever it happens, it gives false confidence to some hapless rat ... er, investor. (I know, because I've been that rat. And maybe you have, too.)

At the end of 2007, you might have been impressed with the 10-year average return of the Winslow Green Growth (FUND: WGGFX) fund, which topped 20%, and with its 92% gain in 2003. Pellets! Pellets! (You might not have paid much attention to its expense ratio, which is a bit on the steep side, at 1.45%, or noticed that it lost more than 37% in 2002.)

If you invested in the fund at the beginning of 2008, you'd be down nearly 27% as of this writing. This fund, recently invested in companies such as Chipotle Mexican Grill (NYSE: CMG), First Solar (Nasdaq: FSLR), and LSB Industries (NYSE: LXU), might continue to do well in the long run. But if you invest in it, study it first.

Avoid financial mistakes
It's never too late to change bad investment habits. Look beyond pellets. Look for great companies with growing sales, earnings, and profit margins, with smart leaders and competitive advantages. Look for outstanding mutual funds with great track records, brilliant managers, low fees, and solid prospects.

I invite you to test-drive our Motley Fool Champion Funds newsletter, where I've found a bunch of winners, myself. Last time I checked, its recommendations were beating the market, on average, by 18 percentage points.

Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article. Disney is a Motley Fool Stock Advisor recommendation. Try our investing services free for 30 days. The Motley Fool is Fools writing for Fools.