Almost half of all financial assets held by households in the 65-and-over category can now be considered in retirement income “drawdown” mode, according to new research.

Roughly $4.3 trillion of the $9 trillion held by households in that age category is being used to draw 4% or more of income, says Hearts & Wallets, a partnership formed by Chris Brown of Sway Research, Newton, N.H., and Laura Varas of Mast Hill Consulting Inc., Hingham, Mass.

In 2006, the researchers point out, the portion of retiree assets in retirement income “drawdown” was only 20%.

The researchers say the main driver of the increase comes from affluent retirees having $1million to $2 million in assets who are taking income at an annual rate of 4% to 6% of investment assets, including returns.

Overall, the retirement income market now represents 18% of U.S. household investable assets, the researchers say.

Hearts and Wallets defines the market for retirement income products and services as “retirees who depend on their personal assets as a primary income source, rather than as a supplement to Social Security and pensions.” These investors logically seek to maximize income over their lifetimes, the researchers say