Although most of today’s retirees are financially secure, they may be living in a golden age that baby boomers and generation Xers largely won’t enjoy when they quit working, researchers say.

Longer retirements and declines in retirement incomes as a percent of preretirement earnings will mean less comfortable retirements for many Americans, the Center for Retirement Research at Boston College says.

The CRR developed a National Retirement Risk Index to measure the share of working-age households at risk of being unable to maintain their preretirement standard of living after they stop working.

Target income is based on a complex formula that considers such factors as income level, number of earning adults in a household, availability of defined benefit pensions and home ownership.

The index defines at-risk households as those who are likely to have retirement income more than 10% below the target needed to maintain their preretirement living standard. It defines retirement income to include the usual sources, such as 401(k)s and defined-benefit plans as well as Social Security. It also includes reverse mortgages on the value of people’s homes.

For example, among all households, the average target income replacement rate would be 73% under the CRR formula. In other words, to continue to live comfortably, the average household would need retirement income equal to 73% of its preretirement income. That target would range from 81% for those in the bottom third of income groups to 67% of those in the top third, according to CRR.

Based on its index, CRR concluded that even if people retire at age 65 and annuitize all their household wealth, 43% would be at risk of having inadequate retirement income.

For early boomers, born from 1946 through 1954, 35% of all households would be at risk. The at-risk share rises to 44% for the late boomers (1955-1964) and 49% for generation Xers (1965-1972), according to the CRR.

Their ominous predictions reflect the trend toward longer retirements and likely declines in retirement incomes compared to preretirement earnings–otherwise known as income replacement rates.

The CRR says it developed the index in the hope it would increase Americans’ awareness of these troubling trends.

It bases the index on a study of about 4,500 nationally representative households known as the Survey of Consumer Finance, conducted every three years by the Federal Reserve and last completed in 2004.

The Index projects where these households would be financially when the senior household member reaches age 65, which the CRR argues is a conservative estimate given today’s lower average retirement age.

In the future, U.S. households in general will have to retire later to have an adequate income, the CRR warns.

One-earner couples, who receive more generous Social Security benefits, are less likely than two-earner couples to be at risk, the CRR points out. This outcome is the result of a Social Security system that provides a 50% spouse’s benefit. When both partners work, they increase pre-retirement earnings but often don’t increase the benefit they would receive upon retirement.

Single women are more likely to be at risk than single men because a greater proportion of single women are in the bottom third of incomes, where the likelihood of being at risk is the highest.

The research team concluded the situation is not hopeless.

For instance, “if people choose to work longer–even just 2 years–and save 3% more, they substantially can improve the outlook for their retirement security,” the researchers concluded.

Nationwide Financial Services Inc., Columbus, Ohio, which helped fund the study, issued a statement supporting the use of the index to gauge Americans’ ability to enjoy a decent retirement.

“For the past decade, we have heard qualitatively that the retirement outlook for Americans is troubling,” said Mark Thresher, president of Nationwide. “Now the risk is quantified by a respected source. It is time we use tools such as the index to inform and spur needed debate on policies that can improve the retirement outlook for Americans.”