WASHINGTON, Dec. 31 (UPI) -- Unrealistically short life expectancy projections for U.S. residents are about to make Social Security's financial problems even worse.

"Life expectancy will make a very big difference in the fiscal viability of Social Security, but the agency's projections of longevity appear too conservative," said demographer Samuel H. Preston of the University of Pennsylvania.

The agency assumed that "past advances in life expectancy are unrepeatable, even though the medical research establishment is routinely producing important breakthroughs that reduce the incidence or fatality of a variety of diseases," he said.

The U.S. National Center for Health Statistics says life expectancy at birth was 47.3 years in 1900, rose to 68.2 by 1950 and reached 77.3 in 2002. The latest annual report of the Social Security trustees projects that life expectancy will increase just six years in the next seven decades, to 83 in 2075.

Social Security says male life expectancy at birth will be 81.2 years in 2075, and female life expectancy will reach 85 years by 2075.