Americans held $4.5 trillion in all employer-based DC retirement plans on Dec. 31, 2011, of which $3.1 trillion was held in 401(k) plans, according to data from the Investment Company Institute. Those figures are up from $4.3 trillion and $2.9 trillion, respectively, as of Sept. 30, 2011.

Mutual funds managed $2.5 trillion of the assets held in 401(k), 403(b), and other defined contribution plans at the end of December, up from $2.3 trillion at the end of September. Mutual funds managed 55% of DC plan assets at the end of the fourth quarter.

Also, as of December 31, 2011, target date mutual fund assets totaled $376 billion, up 9.6% in the fourth quarter and up 10.6% for the year. Retirement accounts held the bulk of target date mutual fund assets: 91 percent of target date mutual fund assets were held through DC plans and IRAs.

The institute also reported total U.S. retirement assets of $17.9 trillion as of December 31, 2011, up 4.9% in the fourth quarter of 2011 and about unchanged for the year. Retirement savings accounted for 36% of all household financial assets in the United States at the end of 2011.

As well, assets in individual retirement accounts totaled $4.9 trillion at the end of 2011, an increase of 4.6% from the end of the third quarter. DC plan assets rose 4.8% in the fourth quarter to $4.5 trillion. Government pension plans—including federal, state, and local government plans—held $4.5 trillion in assets as of December 31, up 6.6% from September 30. Private-sector defined benefit plans held $2.4 trillion in assets at the end of the fourth quarter of 2011, and annuity reserves outside of retirement accounts accounted for another $1.6 trillion.

Hung Tran writes for Money Management Executive.