The Investment Company Institute said that 54.5 million households had investments in equities or bonds through early 2008.

This was 47% of U.S. households, up from 39% in 1989, the first year for which directly comparable survey data were available, the institute said on Tuesday.

Based on a survey of more than 5,000 households, a two-decade rise in equity and bond investment was fueled by the rapid growth of defined contribution retirement savings plans, such as 401(k)s, according to the Washington-based institute.

From 1989 to 2004, the latest year for which comparable data is available, the number of participants in private-sector, defined contribution plans nearly doubled, from 36 million to 65 million.

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