During the past quarter century fertility has dropped below replacement levels in many parts of the world. According to United Nations estimates, in 2005 this was the case in 65 countries, comprising 43 percent of the world's population. In many cases, most notably in Europe and East Asia, the shortfall of fertility from the level that would be necessary in the long run to sustain a stationary population is substantial. In Europe, for example, the average total fertility rate for the period 2000–2005 was 1.4. Indefinite maintenance of such a level implies a shrinkage of the total population by one-third over a generation–roughly every 30 years. Accompanying that rapid decline of total numbers would be an age structure containing a preponderance of the elderly, posing extreme adjustment difficulties for the economic and social system.

Societies that wish to avoid radical depopulation would have to engineer a substantial rise infertility–if not to full replacement level (slightly more than two children per woman), then at least to a level that would moderate the tempo of population decline and make population aging easier to cope with. An additional counter to declining numbers, if not significantly to population aging, could come from net immigration. This is the demographic future assumed in the UN medium-variant projections for countries and regions currently of very low fertility. Thus, for example, in Europe over the period up to 2050 fertility is assumed to rise to 1.85 and net immigration to amount to some 32 million persons. The UN projections also anticipate further improvement in average life expectancy–from its current level of 74 years to 81 years. This factor slows the decline in population size but accelerates population aging. Under these assumptions, Europe's population would decline from its present 728 million to 653 million by 2050. At that time the proportion of the population over age 65 would be 27.6 percent, nearly double its present share.

Demographic change of this nature is not a novel prospect. It was envisioned in a number of European countries and in North America, Australia, and New Zealand in the late 1920s and early 1930s. Concern with the possible economic and social consequences generated much discussion at that time among demographers and social scientists at large and also attracted public attention. Possible policy measures that might reverse the downward trend of fertility were also debated, although resulting in only hesitant and largely inconsequential action.

The article by D. V. Glass reproduced below is an especially lucid and concise treatment of demographic changes under conditions of low fertility and their economic and social implications. It appeared in Eugenics Review (vol. 29, no. 1, pp. 39–47) in 1937 when the author was 26 years old. Glass's line of argument is broadly representative of the main focus of demographic analysis in the mid-1930s on aspects of population dynamics, applying the then still novel analytical tool of the stable population model. It also echoes the work of economists then witnessing the great difficulties capitalist economies faced in adjusting to structural changes in consumer demand and labor supply. While Glass addresses these issues primarily with reference to England and Wales, he sees the issues as affecting all industrialized countries. The Malthusian problem of relentless population growth he persuasively declares to be irrelevant for these countries. The Western world faces the opposite problem: population decline, a trend only temporarily masked by the effects of an age distribution that still has a relatively high proportion of women in the child-bearing ages, reflecting the higher fertility level of the past. A stationary population, Glass cogently argues, is to be welcomed, and he considers the absolute size at which zero growth would be achieved relatively unimportant. In contrast, a continuous population decline would have “thoroughly disastrous” results in an individualist civilization and in “an unplanned economic system.” And, he concedes, somewhat quaintly, that sustained below-replacement fertility would pose a great problem “even in a country in which the means of production were owned communally.” Glass's conclusions about the reversibility of low fertility are as pessimistic as those of most informed observers today. Still, he sees hope in a future “rationally planned civilization” that would “produce an environment in which high fertility and a high standard of life will both be possible.” In this context, high fertility means the level necessary to sustain the population in a stationary state. By present-day standards the level Glass calculates as needed for long-term zero growth is indeed fairly high: 2.87 children per woman. But that figure reflects the fact that, when he wrote, mortality up to age 50 was still fairly high and fertility occurred almost wholly within marriage; it also assumes zero net immigration. In the last 70 years much has changed in each of these three components of population dynamics, both in England and Wales and in the rest of Europe. Still, Glass's commentary remains highly relevant to the discussion of the problems of low fertility today.

David Victor Glass (1911–78) was associated with the London School of Economics throughout much of his scientific career. He followed R. R. Kuczynski as reader in demography in 1945 and became professor of sociology in 1948. His work on demography, population history, and population policy had already made him one of the most influential demographers in pre-World War II Britain. After the war he rose to international prominence through pioneering work on the Royal Commission of Population; through his research on historical demography, the history of demographic thought, and social mobility; and through founding, in 1947, the journal Population Studies, which he edited until his death.