Americans’ chances of reaching extreme old age are much lower than previously thought, new research shows.

The findings by a team of demographers at the social science research organization NORC at the University of Chicago contradict a long-held belief that mortality rates level off above age 80. They also explain why the number of Americans age 100 and above is less than half of what the Census Bureau predicted as recently as six years ago.

The research is based on a new way of accurately measuring mortality among people 80 years of age and older, an issue that has proven remarkably elusive in the past. Instead of using self-reports of age to gauge mortality rates, the new study compared previous projections against real, observed death rates. The work will be significant in arriving at more accurate cost projections for programs such as Social Security and Medicare, which are based in part on mortality rates.

The research, done by Leonid A. Gavrilov and Natalia S. Gavrilova and published in the current edition of the North American Actuarial Journal, is based on highly accurate information about the date of birth and the date of death of more than nine million Americans born between 1875 and 1895. The data is publicly available in the Social Security Administration Death Master File. “It is a remarkable resource that allowed us to build what is called an extinct birth cohort that corrects or explains a number of misunderstandings about the mortality rate of our oldest citizens,” said Leonid Gavrilov.

A stark example of the problem of estimating the number of people over 100 came recently when the U.S. Census Bureau revised sharply downward the number of living centenarians. Six years ago, the bureau predicted that by 2010 there would be 114,000 people age 100 or older. The actual number turned out to be 53,364. The projection was wrong by a factor of two.

The newly published paper, titled “Mortality Measurement at Advanced Ages: A Study of the Social Security Administration Death Master File,” explains the discrepancy and is likely to make a difference in the way mortality projections for the very old are done in the future.

The key finding is straightforward—the rise in death rates that occurs as people grow older is the same for the oldest Americans as for those who are younger. Previous studies had supposed that the mortality rate flattens out above age 80. But the new NORC research reveals that the expected mortality deceleration does not take place.

Anne Zissu, chair of the Department of Business NYC College of Technology/CUNY, said the research provides “an essential tool” for developing models on seniors’ financial assets.

Zissu said the research “will alter our financial approach to this valuation of mortality/longevity risk. Demographers and financiers need to work on this issue together, and their models must adapt to each other.”

The mortality rate for people between the ages of 30 and 80 follows what is called the Gompertz Law, named for its founder, Benjamin Gompertz, who observed in 1825 that a person’s risk of death in a given year doubles every eight years of age. It is a phenomenon that holds up across nations and over time and is an important part of the foundation of actuarial science.

For approximately 70 years, demographers have believed that above age 80 the Gompertz Law did not hold and that mortality rates flattened out. The work done by the Gavrilovs, a husband-and-wife team, reveals that the Gompertz Law holds at least through age 106, and probably higher, but the researchers say mortality data for those older than 106 is unreliable.

The Gavrilovs say the extinct birth cohort of people born between 1875 and 1895, which they built using the Social Security Administration Death Master File, reveals beyond question that the mortality rate of people in that cohort aligns with the Gompertz Law.

“It amazes me that the Gompertz model fits so well nearly 200 years after he proposed it,” said Tom Edwalds, Assistant Vice President of Mortality Research for the Munich American Reassurance Company.

Prior estimates of the number of centenarians in the United States were made in less direct ways that were subject to error. They depended, for example, on people self-reporting their age in the U.S. Census, which is less reliable than having actual birth and death data.

Gavrilov and Gavrilova work at the Center on the Economics and Demography of Aging, one of the Academic Research Centers of NORC. The study is supported by the National Institute on Aging.

The Chicago Actuarial Association has invited the authors to present their new findings at the CAA annual meeting in Chicago on March 13.