After the stock market lost 20% of its value in October 1987, Sam Walton, then one of America's richest men, was unfazed.

In less than a week, the value of his Wal-Mart Stores stock had dropped almost $3 billion, reducing his wealth to a mere $4.8 billion. "It's paper anyway," he told the Associated Press. "It was paper when we started and it's paper afterward."

Given...