Rising corporate equity values helped boost balances in U.S. retirement accounts in the first quarter of 2012, according to the Investment Company Institute. As of March 31, U.S. retirement assets totaled $18.9 trillion, up 6.3% from $17.8 trillion in the previous quarter, an increase driven largely by rising stock market prices, ICI announced today.

Most of the assets were held in individual retirement accounts, which totaled $5.2 trillion, followed by defined contribution and government pension plans, which had $4.8 trillion and $4.7 trillion, respectively, at the end of the first quarter. Private-sector defined benefit plans held $2.5 trillion in assets, and annuity reserves outside of retirement plans accounted for another $1.7 trillion.  

Of the $5.2 trillion in individual retirement accounts, $2.4 trillion, or 46%, was invested in mutual funds.

Of the $4.8 trillion in employer-based defined contribution plans, $3.4 trillion was held in 401(k) plans, up from $3.1 trillion in the fourth quarter of 2011. Mutual funds managed $2.8 trillion of assets held in 401(k), 403(b) and other defined contribution plans at the end of March, or 58% of defined contribution plan assets.

Assets in target date mutual funds totaled $425 billion, up 13% from the previous quarter. The bulk of the assets (91%) were held through defined contribution plans and individual retirement accounts.