This aspect of planetary sociology may be illustrated by the rise and decline of total membership in the human family. We know that Roman civilization passed through a completed cycle of population expansion to an optimum, followed by a catastrophic population decline. Western civilization has been experiencing a population expansion or explosion that can be measured with a moderate degree of statistical accuracy. Planetary human population doubled from 500 million in 1650 to 1000 million in 1850. Between 1850 and 1950 population more than doubled (from 1000 million to 2,500 million). In 1975 the human population of the earth is close to 3,700 million. An essential aspect of world government will be a population program designed to adjust social structure and planning to the means of production and to make generally available to all humans and, where possible, all living things, the results of invention, discovery and experience with affluence, general security and wide variations of vocational and avocational choice. In practice such a program would include the planned utilization and conservation of nature and the conscious improvement of society by society. Social planning at the planetary level could deal chiefly with large national or regional groupings, more or less divergent in viewpoint but conscious of the necessity for bringing local and regional groups together in order to secure common agreement and to take part in directed joint actions. Such efforts must aim at sufficient cohesion to provide for normal social function at all levels; sufficient permissiveness to allow for a measure of self-determination at all levels; sufficient authority to carry on production and distribution at all levels, and sufficient libertarianism to tolerate discussion and opposition at all levels, with a maximum degree of self sufficiency and self-determination at all levels. Nowhere is the need for social planning more in evidence than in the sphere of human population. In the early years of the present twentieth century, the human population was doubling in about 50 years (from 1500 million in 1900 to 2500 million in 1950, from 1,900 million in 1925 to 3,800 million in 1975). Had this rate of growth continued for another hundred years the planet's fertile acres would have been fully occupied by jostling crowds with _standing-room only_ signs in the more desirable living spaces. Japan, the United States, several countries of West Europe and China have launched campaigns to reduce net population increase to one percent per year or less.