In nearly 200 years of recorded stock-market history, no calendar decade has seen such a dismal performance as the 2000s. Investors would have been better off investing in pretty much anything else, from bonds to gold or even just stuffing money under a mattress. Since the end of 1999, stocks traded on the New York Stock Exchange have lost an average of 0.5% a year thanks to the twin bear markets this decade. The period has provided a lesson for ordinary Americans who used stocks as their primary way of saving for retirement. Many investors were lured to the stock market by the bull market that began in the early 1980s and gained force through the 1990s. But coming out of the 1990s, the best calendar decade in history with a 17.6% average annual gain, stocks simply had gotten too expensive. Companies also pared dividends, cutting into investor returns. And in a time of financial panic like 2008, stocks were a terrible place to invest. The decline edges out the 0.2% decline stocks suffered during the Depression years of the 1930s, which up until now held the title of worst decade. And it is worse than other decades with financial panics, such as in 1907 and 1893. Even the 1970s, when a bear market was coupled with inflation, wasn't as bad as the most recent period. The S＆P 500 lost 1.4% after inflation during that decade. That is especially disappointing news for investors, considering that a key goal of investing in stocks is to increase money faster than inflation. 'This decade is the big loser.' said Mr. Jones. For investors counting on stocks for retirement plans, the most recent decade means many have fallen behind retirement goals. Many financial plans assume a 10%0 annual return for stocks over the long term, but over the last 20 years, the S＆P 500 is registering 8.2% annual gains. Should stocks average 10% a year for the next decade, that would lift the 30-year average return to only 8.8%, said North Carolina State's Mr. Jones. It is even worse news for those who started investing in 2000; a 10% return a year would get them up to only 4.4% a year. There were ways to make money in U. S. stocks during the last decade. But the returns paled in comparison with those posted in the 1990s. Of the 30 stocks today that comprise the Dow Jones Industrial Average, only 13 are up since the end of 1999, and just two, Caterpillar Inc. and United Technologies Corp., doubled over the 10-year span. So what went wrong for the U.S. stock market? For starters, it turned out that the old rules of valuation matter. 'We came into this decade horribly overpriced,' said Jeremy Grantham, co-founder of money managers GMO LLC. In late 1999, the stocks in the S＆P 500 were trading at about an all-time high of 44 times earnings, based on Yale professor Robert Shiller's measure, which tracks prices compared with 10-year earnings and adjusts for inflation. That compares with a long-run average of about 16. Buying at those kinds of values, 'you'd better believe you're going to get dismal returns for a considerable chunk of time,' said Mr. Grantham, whose firm predicted 10 years ago that the S＆P 500 likely would lose nearly 2% a year in the 10 years through 2009. Despite the woeful returns this decade, stocks today aren't a steal. The S＆P is trading at a price-to-earnings ratio of about 20 on Mr. Shiller's measure. Mr. Grantham thinks U.S. large-cap stocks are about 30% overpriced, which means returns should be about 30% less than their long-term average for the next seven years. That means returns of just 1.6% a year before adding in inflation. Another hurdle for the stock market has been the decline in dividends that began in the late 1980s. Over the long term, dividends have played an important role in helping stocks achieve a 9.5% average annual return since 1926. But since that year, the average yield on S＆P 500 stocks was r