Peter Bernstein has witnessed just about every financial crisis of the past century.

As a boy, he watched his father, a money manager, navigate the Depression. As a financial manager, consultant and financial historian, he personally dealt with the recession of 1958, the bear markets of the 1970s, the 1987 crash, the savings-and-loan crisis of the late 1980s and the 2000-2002 bear market that followed the tech-stock bubble.

Today's...