Recent advances in genetics, nutrition, and medicine have spurred predictions of nearly limitless increases in human life expectancy over the coming century. However, one respected demographer believes this optimism needs to be balanced with the realities of human politics, history, and biology as our understanding of the complex interactions of social and biological factors that determine mortality levels is still imprecise.
Life expectancy projections typically originate from a set of either demographic or biological assumptions. Demographers rely too often on the assumption that recent historical patterns will repeat themselves over coming decades. However, many of these "trends" are themselves relatively short-lived. For example, when gains in life expectancy began to slow during the 1950's, some demographers jumped to the conclusion that life expectancy had now neared its limit -- even though that slowdown has been reversed through the medical and nutritional advances of the past 40 years. Any projection based on less than 20 years of experience that extends 50 to 100 years into the future is imprudent, if not foolhardy.
If we look back over a longer time period -- the past century -- we instead witness a strong, steady growth in life expectancy among all affluent societies. Average life expectancy will probably continue to increase at this steady (albeit undramatic) pace over the next few decades, reaching about 85 years of age by the mid-21st century. But won't some of the startling new advances in medicine or genetics accelerate that process? One such advance, the recent finding that gene manipulation may extend the life of cells indefinitely, was hailed by some biologists and geneticists as the first step on the road to immortality.
However, previous milestone discoveries -- of germs in 1882, or penicillin in 1928 -- are the root causes of much of the slow, steady increases in life expectancy we benefit from today. So current "breakthroughs" are simply "a continuation of social and technological advance(s) on a par with these earlier achievements, and should trigger no greater extensions in overall life expectancy than did their predecessors.
Finally, history teaches us to be cautious. No demographic projections could have anticipated the rise of mortality in the former Soviet Union, (or) the emergence of AIDS... during the 1980's. And that great gulfs in life expectancy still exist between developed and developing nations. This means that the greatest uncertainties affecting future mortality trends derive from social and political, rather than technological, factors.
Science April 17, 1998;280:395-397