JACKSON HOLE, Wyo. -- Central bankers, academics and scholars at the Federal Reserve Bank of Kansas City's annual conference warned that faster productivity growth, longer working lives and increased immigration won't spare the U.S. and other aging societies from the pain of restructuring pension and health-care programs for older citizens to make those benefits affordable.

Fed Chairman Alan Greenspan set the gloomy tone, warning that the U.S. may have already promised retirees more in Social Security and Medicare benefits than it can deliver, and urged policy makers to "recalibrate" benefits without raising taxes. "If we delay, the adjustments could be abrupt and painful."

In 2001, Mr. Greenspan said the acceleration in underlying U.S. economic growth that he was among the first to spot in the mid-1990s could keep Social Security solvent for much longer than expected. But on Friday, he warned against looking to such growth as salvation: "History discourages the notion that the pace of growth will continue to increase."

Instead, Mr. Greenspan, who is expected to retire in 2006 just short of age 80, believes that more people should follow his example. He noted that life spans have increased, people stay healthy longer and work is less physically demanding than a century ago. Despite this, Americans have been retiring earlier, he said. He predicted this would reverse due to concerns about retirement income and a scarcity of experienced labor. Mr. Greenspan suggested Social Security and Medicare benefits be made less generous, such as through later qualifying ages, to discourage early retirement. He predicted that as "an ever-increasing proportion of the electorate is...essentially barred from continuing to work" because of age, political pressure will mount to permit later retirement.

But the idea has critics. Many employers force older workers to retire to make way for younger, more-productive workers. Harvard University President Lawrence Summers said that since mandatory retirement was abolished for university faculty in 1994, Harvard professors now retire on average at age 72: "I view this trend with terror."

And evidence appears to contradict Mr. Greenspan's prediction of political support for later retirement. Politicians have criticized his previous calls for making benefits less generous, such as pushing back the retirement age. The French ruling party was clobbered in regional elections in March in part over raising the qualifying age for state pensions. Unlike Mr. Greenspan, many people don't like their jobs and look forward to retirement. "It's easy to be a professor at 75; it's not so easy to lift heavy objects," said Princeton University economist Alan Blinder.

The popular belief that rich countries could ease the strains of aging via immigration from younger, developing countries fared badly at the conference. Most participants agreed the level of immigration needed to offset the aging of Western populations is politically impossible.

Others suggested it may be unfair to the developing countries that, thanks to dramatic declines in fertility and mortality, will in a few decades face an age crunch of their own, but with far weaker finances. "For the rich countries to cherry-pick skilled international migrants to finance their own retirement...seems almost unbelievably shortsighted and self-serving," said John Helliwell of the University of British Columbia. Just 4% of Nigerians have more than 12 years of school, compared with 83% of Nigerian immigrants to the U.S., reported Nancy Birdsall of the Center for Global Development.

Mr. Helliwell said that outsourcing, for all the angst it has caused in the U.S., may achieve the same economic benefits of immigration but with far more social harmony: It spreads know-how and wealth in the poor country and minimizes immigration-related strains in the developed country.

Demographics and government retirement benefits, the theme of this year's Jackson Hole conference, are things over which central bankers don't have direct control. Their gloom seemed to reflect a concern that the problems are too large for any democratically elected government to fix.

But the International Monetary Fund's research director, Raghuram G. Rajan, suggested that the gloom may be overdone. Any single solution appears hopeless: The IMF estimates solving the age crunch in the developed world by 2050 would require an 11-percentage-point increase in the share of the population that works, a 30% increase in immigration or a seven-year increase in retirement age. In Japan, more than 100% of the population would need to go to work or the retirement age would have to be raised above the expected life span. "But if you take a combination of these three, it turns out to be far more attractive," he said.