Pushing Life's Limits

　　John R. Wilmoth， a demographer（人口統(tǒng)計學家） at the University of California， Berkeley， has collected a wealth of data on humankind's increasing longevity（壽命） over the past two centuries. And this week in Science（美國《科學》雜志）， he has some good news： The maximum average age we can reach too is on the rise. This finding contradicts the common scientific belief that life span has a biological upper limit of 120 years or so. "Whether 115 or 120 years， it is a legend created by scientists who are quoting each other，" Wilmoth says. （He is shown at right with Christian Mortensen， who died in 1998 at the age of 115.）

　　Working with colleagues in the U.S. and Sweden， Wilmoth scrutinized（仔細審查） the Swedish national death records——considered the best in the world——from 1861 forward. They found that ages at death had shifted upward for 138 years——a trend that accelerated in the 1970s. Although some scientists presumed the larger number of very old people in later years was due simply to a larger population base， the new data show the main cause is actually increased survival after age 70. "We have shown that the maximum life span is changing，" Wilmoth adds. "It is not a biological constant."