Here's something I found after surfing a few links. Given the interest in intelligence augmentation and biological immortality here, I figured you guys would find it useful to know; I wasn't particularly planning on retiring (given biological immortality or uploading technologies), but if this is true, I definitely won't be for as long as I can avoid it.

The two economists call their paper “Mental Retirement,” and their argument has intrigued behavioral researchers. Data from the United States, England and 11 other European countries suggest that the earlier people retire, the more quickly their memories decline.

The implication, the economists and others say, is that there really seems to be something to the “use it or lose it” notion — if people want to preserve their memories and reasoning abilities, they may have to keep active.

“It’s incredibly interesting and exciting,” said Laura L. Carstensen, director of the Center on Longevity at Stanford University. “It suggests that work actually provides an important component of the environment that keeps people functioning optimally.”

While not everyone is convinced by the new analysis, published recently in The Journal of Economic Perspectives, a number of leading researchers say the study is, at least, a tantalizing bit of evidence for a hypothesis that is widely believed but surprisingly difficult to demonstrate.

Researchers repeatedly find that retired people as a group tend to do less well on cognitive tests than people who are still working. But, they note, that could be because people whose memories and thinking skills are declining may be more likely to retire than people whose cognitive skills remain sharp.

And research has failed to support the premise that mastering things like memory exercises, crossword puzzles and games like Sudoku carry over into real life, improving overall functioning.

“If you do crossword puzzles, you get better at crossword puzzles,” said Lisa Berkman, director of the Center for Population and Development Studies at Harvard. “If you do Sudoku, you get better at Sudoku. You get better at one narrow task. But you don’t get better at cognitive behavior in life.”

The study was possible, explains one of its authors, Robert Willis, a professor of economics at the University of Michigan, because the National Institute on Aging began a large study in the United States nearly 20 years ago. Called the Health and Retirement Study, it surveys more than 22,000 Americans over age 50 every two years, and administers memory tests.

That led European countries to start their own surveys, using similar questions so the data would be comparable among countries. Now, Dr. Willis said, Japan and South Korea have begun administering the survey to their populations. China is planning to start doing a survey next year. And India and several countries in Latin America are starting preliminary work on their own surveys.

“This is a new approach that is only possible because of the development of comparable data sets around the world.” Dr. Willis said.

The memory test looks at how well people can recall a list of 10 nouns immediately and 10 minutes after they heard them. A perfect score is 20, meaning all 10 were recalled each time. Those tests were chosen for the surveys because memory generally declines with age, and this decline is associated with diminished ability to think and reason.

People in the United States did best, with an average score of 11. Those in Denmark and England were close behind, with scores just above 10. In Italy, the average score was around 7, in France it was 8, and in Spain it was a little more than 6.

Examining the data from the various countries, Dr. Willis and his colleague Susann Rohwedder, associate director of the RAND Center for the Study of Aging in Santa Monica, Calif., noticed that there are large differences in the ages at which people retire.

In the United States, England and Denmark, where people retire later, 65 to 70 percent of men were still working when they were in their early 60s. In France and Italy, the figure is 10 to 20 percent, and in Spain it is 38 percent.

Economic incentives produce the large differences in retirement age, Dr. Rohwedder and Dr. Willis report. Countries with earlier retirement ages have tax policies, pension, disability and other measures that encourage people to leave the work force at younger ages.

The researchers find a straight-line relationship between the percentage of people in a country who are working at age 60 to 64 and their performance on memory tests. The longer people in a country keep working, the better, as a group, they do on the tests when they are in their early 60s.

The study cannot point to what aspect of work might help people retain their memories. Nor does it reveal whether different kinds of work might be associated with different effects on memory tests. And, as Dr. Berkman notes, it has nothing to say about the consequences of staying in a physically demanding job that might lead to disabilities. “There has to be an out for people who face physical disabilities if they continue,” she said.

And of course not all work is mentally stimulating. But, Dr. Willis said, work has other aspects that might be operating.

“There is evidence that social skills and personality skills — getting up in the morning, dealing with people, knowing the value of being prompt and trustworthy — are also important,” he said. “They go hand in hand with the work environment.”

But Hugh Hendrie, an emeritus psychology professor at Indiana University School of Medicine, is not convinced by the paper’s conclusions.

“It’s a nice approach, a very good study,” he said. But, he said, there are many differences among countries besides retirement ages. The correlations do not prove causation. They also, he added, do not prove that there is a clinical significance to the changes in scores on memory tests.

All true, said Richard Suzman, associate director for behavioral and social research at the National Institute on Aging.

Nonetheless, he said, “it’s a strong finding; it’s a big effect.”

If work does help maintain cognitive functioning, it will be important to find out what aspect of work is doing that, Dr. Suzman said. “Is it the social engagement and interaction or the cognitive component of work, or is it the aerobic component of work?” he asked. “Or is it the absence of what happens when you retire, which could be increased TV watching?”

“It’s quite convincing, but it’s not the complete story,” Dr. Suzman said. “This is an opening shot. But it’s got to be followed up.”