American retirement assets grew 6.7% in 2007, according to the mutual funds trade group Investment Company Institute. Fuelled by strong growth in individual retirement accounts (IRAs) and employer-sponsored defined contribution plans such as 401(k) accounts, retirement account assets topped $17.6 trillion at the end of last year.  

   Among the result from the 2008 Investment Company Fact Book:

• IRAs and defined contribution plans constituted $9.2 trillion, or roughly half, of total retirement savings.
• Mutual funds managed $4.6 trillion or 26% of retirement-market assets. Pension funds, insurance companies and brokerage firms managed the remaining $13 trillion.
• Net cash flows into increasingly popular lifestyle and lifecycle funds reached a record $92 billion last year.
• More than three-quarters of 401(k) assets invested in stock funds were in funds with expense ratios of less than 1%.