Warnings set forth by many scientists and popular pundits that an overcrowded, poisoned, and exhausted earth lies in wait for our grandchildren, perhaps even for our children, and our own old age are shrouded in controversy. Technological optimists, pointing to the accomplishments of the past, look for new materials and sources of energy; economic optimists, pointing to the amazing records of past growth and to the theoretical efficiencies of perfect competition, look for the market economy to induce appropriate technological changes and resource substitutions; social and political optimists, pointing to mankind's seemingly limitless adaptability, look for government policies, social reorganization, and modifications of individual behavior to alleviate problems as they arise.

At various times and places in the world, however, local situations have approximated on a relatively small scale the conditions warned of by contemporary Cassandras. Certain well-known cities of the world have long been regarded as hideously overcrowded. Others have for short periods experienced alarming death rates from polluted air and water. The energy crisis has brought the potential effects of resource exhaustion home to people everywhere, and rapid, seemingly uncontrollable inflation reminds us that the stable development of complex economies can scarcely be taken for granted. Even if – as some argue – crash programs for resource conservation, pollution abatement, and population control do not yet seem warranted, it is long since obvious that a better understanding of the global state of mankind is needed and that improved methods for projecting long-run development should receive high-priority attention.