Biological plausibility

In a recent review in Science, Oeppen and Vaupel (1) documented the steady linear growth in life expectancy over the past 160 years, increasing by approximately 2.5 years for each decade between 1840 and 2000. Oeppen and Vaupel argue that there is no evidence that the rate of increase is slowing down; that is, there are no demographic data on which to estimate a "maximum" human life span achievable at some future date. If these same trends continue, then average life span for both women and men will exceed 100 years later in this century.

A commonly used test of an epidemiologic finding is biological plausibility—does the reported finding "make sense," given our current understanding of how the body works? This is not an infallible test. We can sometimes be misled by its application. But a lack of biological plausibility tends to stimulate a rigorous search for potential biases that might have produced a spurious association. In similar fashion, demographic analyses and projections should have clinical plausibility.

Much of the public discussion about the aging boom proceeds from the unspoken assumption that the old people of tomorrow will be pretty much the same as the old people of today. When projections are made of 4 million United States centenarians in 2050, the discussions seem to assume that those 4 million people will be just like the 80,000 centenarians alive today. To give one example, projections for future prevalence of Alzheimer's disease and other chronic illnesses account for projected increases in life expectancy but assume no change in age-specific incidence of the disease.

Such assumptions are not clinically plausible. Life expectancy increases because people are healthier. What does "healthier" mean? I think most clinicians would assume that a population becomes healthier because the prevalence of acute and chronic diseases decreases, for whatever reasons. This, in turn, produces longer lives. Thus, as life expectancy increases, individuals at any given age—say age 75—will look younger than did 75-year-olds of an earlier era.

Since 1960, life expectancy has increased by approximately 10 years. One should then assume that the average 75-year-old today looks like the average 65-year-old of 1960. Similarly, if we want to picture the 4 million centenarians in 2050, we should think of our current patients in their late 80s, while people in their late 80s in 2050 might resemble those who are 75 today.

Everything I have stated above is a massive oversimplification. While it is an oversimplification, I think it explains at least 80% of the relationship between increasing life expectancy and patterns of health and disability. Robine and Michel have provided the tools to move beyond that oversimplification, and similar oversimplifications by other authors, to try to come to a more complete understanding of how increasing life spans might affect rates of morbidity and disability. What Professors Robine and Michel (2) have done in their thoughtful discussion of population aging is to systematically work through how different clinically plausible theories would affect patterns of disability, and then test those theories against the available evidence. They have identified four elements that affect the disability/mortality relationship. This should assist further discussions. What I found particularly interesting is their emphasis on cultural determinants of health and disability. Both health and disability have a subjective component, and are thus affected by beliefs and expectations. These vary regionally and temporally, as well as by individual characteristics. Thus, "good health" is an interaction of an individual's physiologic state with his or her own expectations of what good health should feel like. Forecasting health and disability then takes on some of the same uncertainties as economic forecasts, which depend so much on individuals' beliefs and behaviors. That subjective component means that, just as in economics, we will always be much better in explaining what has happened than describing what will happen.

Caring for the Earth

This strategy is founded on the conviction that people can alter their behaviour when they see that it will make things better, and can work together when they need to. It is aimed at change because values, economies and societies different from most that prevail today are needed if we are to care for the Earth and build a better quality of life for all.

Over a decade ago our organizations published the World Conservation Strategy. It stated a new message: that conservation is not the opposite of development. It emphasized that conservation includes both protection and the rational use of natural resources, and is essential if people are to achieve a life of dignity and if the welfare of present and future generations is to be assured. It drew attention to the almost limitless capacity of people both to build and destroy. It called for globally coordinated efforts to increase human well-being and halt the destruction of Earth's capacity to support life.

The World Conservation Strategy and its successors

The World Conservation Strategy was published in 1980. It emphasized that humanity, which exists as a part of nature, has no future unless nature and natural resources are conserved. It asserted that conservation cannot be achieved without development to alleviate the poverty and misery of hundreds of millions of people. Stressing the interdependence of conservation and development, the WCS first gave currency to the term "sustainable development".

Sustainable development depends on caring for the Earth. Unless the fertility and productivity of the planet are safeguarded, the human future is at risk. The World Conservation Strategy therefore emphasized three objectives:

* essential ecological processes and life-support systems must be maintained;
* genetic diversity must be preserved;
* any use of species or ecosystems must be sustainable.

Since 1980, the World Conservation Strategy has been tested by the preparation of national and subnational conservation strategies in over 50 countries. In 1987, in its report Our Common Future, the World Commission on Environment and Development advanced our understanding of global interdependence and the relationship between economics and the environment. It contributed significantly to the growing recognition of the need for sustainable development and international equity. Also in 1987, governments adopted an Environmental Perspective to the Year 2000 and Beyond, which defined a broad framework to guide national action and international cooperation for environmentally sound development. In June 1992 they will meet in Rio de Janeiro to agree an agenda for environment and development in the 21st Century.

In the decade since 1980 the complexity of the problems we face has become clearer, and the need to act has become more pressing. In this new document we set out the broad principles, and an array of consequent actions, upon which we believe the future of our societies depends.

We accept that the actions called for in this Strategy will not be taken easily. Inertia is strong within human societies. Governments have to balance the gains of change against the inevitable costs of upheaval, and tend to develop policies through a succession of cautious steps. People cling to what they have, especially if they perceive that change threatens their personal power and wealth. It will be difficult for many communities to switch resources from war to peace, national to global advantage, or immediate gain to future welfare. But the conflicts, famine and strife that persist in an over-stressed world show how essential it is to seek a new approach. This reinforces our conviction that this Strategy must go ahead.

Caring for the Earth has been prepared through a wider process of consultation than was possible when we wrote the World Conservation Strategy a decade ago. It is intended to re-state current thinking about conservation and development in a way that will inform and encourage those who believe that people and nature are worth caring about and that their futures are intertwined. It is also intended to persuade people at all levels that they can do something, or help cause something to be done, that will lead to better care for the Earth.

The actions of our organizations and others will have to be reshaped if we are to ensure speedy and efficient implementation of this Strategy. We urge all governments, intergovernmental organizations, non-governmental groups, and individuals to help achieve that essential goal.