Behavioral-finance theory also helps explain why many people refuse to join a 401(k) savings plan at work, even when their company matches their cont… - Burton Malkiel

The castle-in-the-air theory of investing concentrates on psychic values. John Maynard Keynes, a famous economist and successful investor, enunciated the theory most lucidly in 1936. It was his opinion that professional investors prefer to devote their energies not to estimating intrinsic values, but rather to analyzing how the crowd of investors is likely to behave in the future and how during periods of optimism they tend to build their hopes into castles in the air. The successful investor tries to beat the gun by estimating what investment situations are most susceptible to public castle-building and then buying before the crowd.

Our survey of historical bubbles makes clear that the bursting of bubbles has invariably been followed by severe disruptions in real economic activity. The fallout from asset-price bubbles has not been confined to speculators. Bubbles are particularly dangerous when they are associated with a credit boom and widespread increases in leverage both for consumers and for financial institutions.

As a random walker on Wall Street, I am skeptical that anyone can predict the course of short-term stock-price movements, and perhaps we are better off for it. I am reminded of one of my favorite episodes from the marvelous old radio serial I Love a Mystery. This mystery was about a greedy stock-market investor who wished that just once he would be allowed to see the paper, with its stock-price changes, twenty-four hours in advance. By some occult twist his wish was granted, and early in the evening he received the late edition of the next day’s paper. He worked feverishly through the night planning early-morning purchases and late-afternoon sales that would guarantee him a killing in the market. Then, before his elation had diminished, he read through the remainder of the paper—and came upon his own obituary. His servant found him dead the next morning.
Because I, fortunately, do not have access to future newspapers, I cannot tell how stock and bond prices will behave in any particular period ahead. Nevertheless, I am convinced that the moderate long-run estimates of bond and stock returns presented here are the most reasonable ones that can be made for investment planning decades into the twenty-first century. The point is not to invest with a rearview mirror projecting double-digit returns from the past into the future. We are likely to be in a low-return environment for some time to come.