Wealthy Americans now hold more assets in individual retirement accounts than they do in workplace-based retirement arrangements such as 401(k) plans and 403(b) plans.

Ownership of both types of retirement accounts is down since 2006, but ownership of workplace-based retirement accounts has fallen much more dramatically than for IRAs, according to Cogent Research L.L.C., Cambridge, Mass.

Affluent Americans now have 31% of their assets in IRAs and 25% in employer-sponsored retirement plans, Cogent says

Much of the money that investors once kept in employer plans is now in IRAs and, to a lesser extent, bank accounts, Cogent says.

Cogent is basing those conclusions on results from a national survey of 4,000 affluent and high net-worth U.S. residents.

IRA penetration has fallen just 5% since 2006, while the number of workplace-based retirement accounts has fallen 23%, Cogent reports.

About 45% of the affluent investors who still had assets in employer plans in 2009 planned to roll the assets into IRAs, up from 39% in 2008, Cogent says.