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Longevity risk—that is, the risk of outliving your retirement savings—is among retirees' biggest worries these days. The Obama administration is trying to nudge employers to add a special type of annuity to their investment menus that addresses that risk. But here's the response they're likely to get: "Meh."

The U.S. Treasury released rules earlier this month aimed at encouraging 401(k) plans to offer "longevity annuities"—a form of income annuity in which payouts start only after you reach an advanced age, typically 85.

Longevity annuities are a variation of a broader annuity category called deferred income annuities. DIAs let buyers pay an initial premium—or make a series of scheduled payments—and set a date to start receiving income.

Some forms of DIAs have taken off in the retail market, but longevity policies are a hard sell because of the uncertainty of ever seeing payments. And interest in annuities of any sort from 401(k) plan sponsors has been weak.

The Treasury rules aim to change that by addressing one problem with offering a DIA within tax-advantaged plans: namely, the required minimum distribution rules (RMDs). Participants in workplace plans—and individual retirement account owners—must start taking RMDs at age 70 1/2. That directly conflicts with the design of longevity annuities.

The new rules state that so long as a longevity annuity meets certain requirements, it will be deemed a "qualified longevity annuity contract" (QLAC), effectively waiving the RMD requirements, so long as the contract value doesn't exceed 25% of the buyer's account balance or $125,000, whichever is less. (The dollar limit will be adjusted for inflation over time.) The rules apply only to annuities that provide fixed payouts—no variable or equity-indexed annuities allowed.

But 401(k) plans just aren't all that hot to add annuities—of any type. A survey of plans this year by Aon Hewitt, the employee benefits consulting firm, found that just 8% offer annuity options. Among those that don't, 81% are unlikely to add them this year.

Employers cited worry about the fiduciary responsibility of picking annuity options from the hundreds offered by insurance companies. Another key reason is administrative complication should the plan decide to change record keepers, or if employees change jobs.

"Say your company adds an annuity and you decide to invest in it—but then you shift jobs to an employer without an annuity option," says Rob Austin, Aon Hewitt's director of retirement research. "How does the employer deal with that? Do you need to stay in your former employer's plan until you start drawing on the annuity?"

Employees are showing interest in the topic: A survey this year by the LIMRA Secure Retirement Institute found 80% would like their plans to offer retirement income options. The big trend has been adding financial advice and managed account options, some of which allow workers to shift their portfolios to income-oriented investments at retirement, such as bonds and high-dividend stocks. Some 52% of workplace plans offered managed accounts last year, up from 29% in 2011, Aon Hewitt reports.

"The big difference is the guarantee," says Austin. "With the annuity, you know for sure what you are going to get paid. With a managed account, the idea is, 'Let's plan for you to live to the 80th percentile of mortality, but there's no guarantee you'll get there.'"

Outside 401(k)s, the story is different. Some forms of DIAs have seen sharp growth lately as more baby boomers retire. DIA sales hit $2.2 billion in 2013, more than double the $1 billion pace set in 2012, according to LIMRA, an insurance industry research and consulting organization. Sales in the first quarter this year hit $620 billion, 55% ahead of the same period of 2013.

Three-quarters of those sales are inside IRAs, LIMRA says, since taking a distribution to buy an annuity triggers a large, unwanted income tax liability. But the action—so far—has been limited to DIAs that start payment by the time RMDs begin. The new Treasury rules could accelerate growth as retirees roll over funds from 401(k)s to IRAs.

"For some financial advisers, this will be an appealing way to do retirement income planning with a product that lets them go out past age 70 1/2 using qualified dollars," says Joe Montminy, assistant vice president of the LIMRA Security Retirement Institute. "For wealthier investors, [shifting dollars to an annuity] is also a way to reduce overall RMD exposure."

Could the trend spill over into workplace plans? Austin doubts it. "I just don't hear a major thirst from plan sponsors saying this is something we should have in our plan."

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