U.S. pension plans, 401(k) plans, individual retirement accounts and other retirement savings vehicles ended 2007 with $15 trillion in assets, up from $7.9 trillion in 1997.

Consultants at Watson Wyatt Worldwide, Washington, have published those figures in a global pension asset study.

Pension assets represent a form of wealth, rather than a flow of income or revenue, but the consultants note that U.S. retirement assets now exceed the $14 trillion U.S. gross domestic product, or national income total, for the first time.

U.S. retirement assets have been growing at a compound annual rate of 6.7% since 1997, while GDP has been growing 4.8% per year over the same period, the consultants report.

The percentage of portfolios held in bonds has fallen to 23%, from 33%, while the percentage in hedge funds, real estate, commodities and other “alternative investments” has increased to 17%, from 9%.