The collective nest egg in the U.S. expanded to a record $16.6 trillion as of March 30, 2007, up from $16.4 trillion at year-end 2006, according to the Investment Company Institute. To put that in context, American retirement assets totaled $9 trillion at year-end 1997.

    The results are the most recent available, according to ICI, a mutual fund trade association that formerly published only year-end figures on retirement savings but has now begun to publish them on a quarterly basis.

    Among the findings, retirement savings comprise nearly 40% of all household financial assets in the United States. Ten years ago, that figure was roughly 30%.

    IRAs comprised the largest chunk of retirement assets with more than $4.3 trillion in the first quarter of this year, up from $4.2 trillion at year-end 2006. Mutual funds manage 47 percent of IRA assets.

    The next biggest categories-government pension plans and employer-based defined contribution plans-each had about $4.2 trillion. Regarding the latter, Americans held $2.75 trillion in 401(k) plans, up slightly from the year-end $2.7 trillion.
    Mutual funds managed $2.16 trillion in assets in 401(k), 403(b), and other defined contribution plans in the first quarter, up from $2.10 trillion at year-end 2006. Mutual funds manage 52% of defined-contribution assets.

    Lifecycle funds, whose managers automatically readjust their ratio of equity to fixed-income holdings from year to year as shareholders age, was the fastest-growing segment with $133 billion at the end of the first quarter of 2007 versus $114 billion at year-end. Almost 90% of assets in lifecycle funds are held in retirement accounts.

    Lifestyle funds, which mix equity and fixed-income investments to maintain a predetermined risk level, managed $204 billion on March 30, up from $189 billion on December 31. Almost half of lifestyle fund assets are in retirement accounts.