Past returns are no guarantee of future success. Just like smokers ignoring the Surgeon General's warning on the side of cigarette packs, investors overlook the most obvious caution about the stock market at their peril.

Even after Friday's stumble over renewed fears about Europe, the Standard & Poor's 500-stock index has gained 7% so far this year, and the Russell 2000 small-stock index is up 11%.

Why...