When the federal government assesses the long-term financial problems of Social Security, it assumes that increases in life expectancy will be slow and measured. But many population experts say they believe that Americans' life expectancy will increase rapidly in the 21st century, making the program's financial problems even worse.

President Bush and Congress are preparing for a debate over the future of Social Security, whose solvency depends not only on factors including productivity, inflation and birth rates but also on how long beneficiaries will be living.

Life expectancy at birth increased by 30 years in the last century, and many independent demographers, citing the promise of biomedical research and the experience of some other industrialized countries, predict significant increases in this century. The Social Security Administration foresees a much slower rise.

"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 Prof. Samuel H. Preston of the University of Pennsylvania, one of the nation's leading demographers.

Dr. Preston said 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."

In a trend that has surprised many experts, the federal welfare rolls have declined over the last three years, even as unemployment, poverty and the number of food stamp recipients have surged in a weak economy.

After Congress overhauled the nation's welfare system in 1996, the number of families receiving benefits dropped much faster than federal and state officials had expected. Even more remarkable, officials say, the rolls did not grow during the recession of 2001 or the sluggish economy since.