Sept. 11, 2001, had only a modest effect on the economy, but the shadows cast over the minds of investors still haven't lifted. The terrorist attacks set the stage for a decade of setbacks that have led many people to regard the stock market as a lost cause.

Ever since, investment portfolios have seemed to be at the mercy of what professional investors call "macro" forces: natural disasters, geopolitical shocks and sudden, systemic failures of stock markets and national economies, from Hurricane Katrina and the financial...