Researchers at the social science research organization NORC at the University of Chicago are challenging the common belief that the U.S. mortality rate flattens out above age 80.

Their findings also provides insight as to why there are only half as many people in the U.S. age 100 and above, which is lower than the Census Bureau predicted six years prior.

Researchers Leonid Gavrilov and Natalia Gavrilova based their findings on extremely accurate data about the date of birth and the date of death of more than nine million American born between 1875 and 1895. The data can be publicly viewed 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 Gavrilov, in a statement.

Six years ago the U.S. Census Bureau predicted that by 2010 there would be a sum of 114,000 people age 100 or older, however, the actual number came to be substantially lower at 53,364. Researchers found that the rate of mortality growth with the age of the oldest Americans is identical to those who are younger, which confirms that the mortality rate flattens out above age 80 does not take place.

Researchers believe these results will be significant in shaping more accurate cost estimates for programs, such as Medicare and Social Security, which are based partially on mortality rates.

The mortality rate for people between the ages of 30 and 80 follows what is known as the Gompertz Law, which is named after its creator Benjamin Gompertz. In 1825, Gompertz observed that a persons risk of death in a given year doubles every eight years of age. Amazingly, the data has held up throughout time and is a key part of actuarial science.

The Gavrilovs believe the extinct cohort of people born between 1975 and 1895, which they built using the Social Security Administration Death Master File, confirms that the mortality rate of people in that cohort match up perfectly with the Gompertz Law.

Tome Edwalds, the Assistant Vice President for the Munich American Reassurance Company said,“It amazes me that the Gompertz model fits so well nearly 200 years after he proposed it. I like the approach of using extinct cohorts methods on SSA DMF (Social Security Administration Death Master File) data by month and the use of male-female ratios to test the quality of the data at advanced ages.”

The study is published in the current edition of the North American Actuarial Journal.