18 percent

The wealth of American families plunged nearly 18% in 2008, erasing years of sharp gains on housing and stocks and marking the biggest loss since the Federal Reserve began keeping track after World War II.

The Fed said Thursday that U.S. households' net worth tumbled by $11 trillion -- a decline in a single year that equals the combined annual output of Germany, Japan and the U.K. The data signal the end of an epoch defined by first and second homes, rising retirement funds and ever-fatter portfolios.

Past downturns have been mere blips compared with the losses Americans faced last year, which set them back to below 2004 levels. "In the postwar period, we've never had anything other than very modest declines. That life experience led many people to think that houses were a one-way bet," says Douglas Cliggott, the chief investment officer of Dover Management LLC.

The decline in Americans' net worth, which was the first in six years, follows an extraordinary boom. Not accounting for inflation, household wealth more than doubled from 1990 to 2000, and then, after a pause, rose nearly 50% before the bust of 2008.