NEW HAVEN, Connecticut -- Living a long time is one of our deepest wishes, and medical and economic progress offers the hope that it will be fulfilled. Some scientists say that the average human life span could reach 90 years or more by midcentury. But what if our wish is granted? What good is a longer life if we cannot maintain our standard of living?

The fundamental difficulty in planning for enhanced longevity is that we do not know whether it will really happen. Life expectancy might well be only 80 years by midcentury -- about where it is now in advanced countries -- if medical progress is disappointing or is offset by new threats or hazards. If we make provisions for long lives that are cut short, we will have wasted huge amounts of precious economic resources. But if we fail to make provisions for lives that are longer, many elderly people will be condemned to poverty.

The whole direction of any country's economy depends on longevity. A huge number of elderly people would mean a lot of people wanting to live in certain locations, seeking certain kinds of living quarters and consuming certain kinds of services. The types of corporations that will succeed, the buildings that are constructed, and the research and development that will be needed all depend on the demographics of demand.