Here’s a scary thought. The stock market doesn’t crash. It just languishesr for the next five or 10 years, moving up and moving down, but in the end barely matching inflation.

This is far from impossible. Based on historical data, Robert Shiller of Yale University and John Campbell of Harvard University predict the reward for investing in the U.S. stock market over the next decade, after adjusting for inflation, will be roughly zero. And they aren’t kooks. Both were among a handful of economists summoned to the Federal Reserve Board just a week before Chairman Alan Greenspan’s admonition about "irrational exuberance."

The stock market has had disappointing decades before. An investment in the 500-share Standard & Poor’s index in 1965 lost 4.25% annually over the subsequent decade, after adjusting for inflation. A 1972 investment lost an annual 3.75% over the next 10 years.

The expectation that stocks will outperform other investments also drives talk about putting Social Security funds in the market. All three factions of the federal advisory panel that reported last week relied on experts’ assurances that, over the long run, stocks will grow about 7% a year faster than inflation while government bonds will climb only 2.3% faster. Over the very long run, history is reassuring — a relevant consideration for 40-year-olds setting aside money for retirement. Columbia University’s Stephen Zeldes, in an observation noted approvingly by the Social Security panel, says that since 1926 there never has been a 22-year period in which stocks have failed to outperform Treasury bonds.

The essence of the Shiller and Campbell argument is captured in the chart be1ow. When the ratio of corporate earnings to stock prices is low — another way of saying that stock prices are high — the market tends to be a poor investment over the next 10 years. Each dot represents the stock market in one year; the farther left the dot, the higher the market. The lower the dot, the worse the market performed in the ensuing decade. "I have this feeling that history repeats itself," Mr. Shiller says.

Using the past decade of corporate earnings to gauge stocks’ value, an unconventional approach that Mr. Shiller uses to avoid distortions from using shorter periods, stocks today are higher than they have been since 1929. Analyses based on dividend yields and book values of corporate assets tell a disturbingly similar tale.

It is worth remembering that economists have a spotty record when it comes to the stock market. One of the century’s greatest economic thinkers, Irving Fisher, a lifelong Yalie, assured investors shortly before the 1929 crash that stock prices had reached "a permanently high plateau." He followed his own advice and lost a substantial fortune. Yale had to buy his house to stop the bank from evicting him.

Mr. Shiller isn’t quite so sure about the danger. "I’ve pulled most of my money out of the stock market," he says. "I’m not completely out. I don’t have complete confidence in this." The U.S. could be on the threshold of an era of rapidly growing productivity and profits, he suggests. Or so many people may be willing to put money into the stock market that it will keep rising.

Perhaps history doesn’t repeat itself. Merton Miller, the University of Chicago Nobel laureate, puts little faith in Mr. Shiller’s projections. The stock market’s historical tendencies, if any, can be overwhelmed by unpredictable events. "The world is too uncertain. Strange things happen," he says. "Who knows what’ll happen in Russia or China or Japan?" If Fed policy had been wiser in the late 1920s, Mr. Fisher might have been right. "Is it true that what goes up goes down? It’s by no means clear," Mr. Miller says.