While the stock market often seems to behave like a manic-depressive who's been off his medication, in fact it's quite rational -- most of the time. Information about the economy and the prospects of specific companies comes in, and the market reacts. Sometimes those reactions are extreme, but they usually sift down to a handful of causes. As legendary investor Warren Buffett likes to say, "Over the short term the market is a voting machine. Over the long term, it's a weighing machine."

So why does the market seem so erratic? Because life in general is unpredictable. A war here, a hurricane there. These things can occur without much warning, having effects on the economy that no one could anticipate.

What's harder to explain is why the market can ignore obvious problems for a long time and then suddenly overreact. Here's the reason: Investors have a hard time gauging the magnitude of problems. Take the dramatic reaction to the Asian crisis in 1997 and the tumult that followed in 1998. Though the experts knew that Asian banks had been overextended for years, few realized how serious the problem was until Thailand devalued its currency in the summer of 1997. Suddenly investors reassessed, and the market took a 544-point, one-day dive -- only to recover most of that ground the very next day. Likewise, when the Russian government, which everyone knew was teetering, defaulted on its debt a year later, the market was thrown into another tailspin.

But if you ignore the occasional surprises that roil the market and focus instead on its long-term behavior, you'll find three factors are key:

Earnings growth.
Over periods of five years or more, stock prices closely track corporate profit growth. And the longer the stretch of time, the more important earnings trends are. Indeed, since World War II, an estimated 90 percent of the stock market's gain has come from profit growth. (That's where Buffett's weighing machine comes in.) As profits add up over time, the scale tips and prices rise, regardless of how investors have voted in any given day, month or year.

Interest rates.
In the short run, changes in interest rates can be more important than earnings. When rates go up, all other things being equal, investors tend to pull money out of stocks and put it into bonds and other fixed-income investments because the returns there are so attractive. That brings stock prices down, and sends bond prices higher. On the other hand, when interest rates come down again, once more with other things equal, then investors tend to shift money into stocks, reversing the previous trend. Note, however, that the operative phrase above is "other things equal." In real life, other things are rarely equal, and so this relationship -- while true in general terms -- is hardly perfect.

Money flows.
Demographics, tax laws and savings patterns all affect the rate at which money flows into stocks (these are a few of the "other things"). That can raise or depress stock prices. The best example in the past decade has been the growth of 401(k) accounts. As baby boomers took advantage of these and other tax-deferred retirement havens to shore up their inadequate savings, the flow of money into mutual funds -- where most 401(k) assets reside -- gave stocks an extra boost.

Next: Bonding with bonds