Even the lucky people who have cash to invest are often cynical about the stock market. "The game is rigged. Smart money vs. dumb money. Insiders win. Efficient markets. You can't beat the pros. Why try?" Etc. Wrong! To be a successful investor, you only need to know one thing.

And here it is, the secret to beating the Wall Street pros: 1) Leave your investments alone for a long time.

That's it. Sure, there are a few supporting details, like "invest in low cost index funds" and "diversify" and "don't think you're some stock picking genius, you fool, you're not." But the essential rule is: just leave your money alone. Don't mess with it. Don't look at it. Don't think about it. Check back in, oh, five or ten years. Or, hell, twenty years. And guess what: the odds are overwhelmingly in your favor. You will have been a successful investor.

Professional investors have to make money on their investment every year. They don't have a real job, for backup; their investment money is what supports them. Making money on investments every year is exceedingly difficult. Most professional investors cannot do it. You, the average schmuck, do not have to do it. You have a real job. You get a paycheck. You're investing for the long haul. And in the long haul, historically, the stock market has almost always gone up.

Here is a very straightforward explanation of this dynamic, on The Motley Fool. (Please see the nifty accompanying chart.) Over a period of three months, stocks have historically gone up only 60% of the time. This is the time frame that professional investors work in. Over a period of ten years, stocks have gone up (accounting for inflation) 88% of the time. Over twenty years, they've gone up 100% of the time. The conclusion: if you hold your stocks for a short period of time, you are gambling, and you may well lose. If you hold your stocks for a long period of time, you will very likely win. Professional investors can't do this. You can. You win!*

*Assuming that a) you have money to invest, b) you invest in low cost broad-based stock index funds, and c) the future resembles the past, which is the best bet to make, since the past is all we have to go on, but which, of course, could fail to be true. You can't predict that either, though, so go ahead and invest, or inflation is sure to eat you alive.

[Motley Fool via Ritholtz. Image via]