The apparent slowing of the world's population growth contains sad as well as good news. The good news is the welcome decline in fertility. The sad news concerns mortality, the second element entering into the calculation of population growth rates.

The population growth rates are slowing not only because people are being born less frequently, but also because deaths are not declining so rapidly. The slackening of the decline in mortality means that the presently observed population-growth slowdown involves significant costs as well as benefits, costs in terms of human welfare that are concentrated especially in the Third World.

What is now appearing is a slowing in the notable health improvements experienced by the world in general and by the developing countries in particular during the years immediately following World War II. In the mid-1950s, world life expectancy was increasing by two-thirds of a year annually. Since then, progress has slowed steadily. By the early 1970s, only minimal gains were being recorded by the developed countries -- which were reaching levels of longevity thought to lie near the biological maximum -- and by those particularly advanced developing countries of Latin America and Asia that were approaching Western standards. While some slackening in the increase in life expectancy was to be expected in those countries, significant slow-downs were also observed where they should not have occurred: in large parts of Asia and Latin America with much less favorable mortality conditions; and possibly also in some regions of high-mortality Africa, an area that had in general failed to participate in the rapid progress earlier enjoyed elsewhere.

For the less developed world as a whole, the rate of life-expectancy improvement declined by nearly one-half, from over two-thirds of a year annually in the mid-1950s to, it is estimated, just over one-third of a year annually in the early 1970s. This happened even though life expectancy for developing countries overall had by then progressed only half of the way from its pre-war starting point to the levels currently enjoyed by the advanced nations.

Increasing recognition of these factors has led demographers to realize that their earlier mortality projections were overly optimistic. "It seems," as United Nations Population Division Director Leon Tabah has written, "that almost everywhere in the Third World the experts were overly hasty in proclaiming an illusory victory over death toward the end of the 1960s." Prepared in happier days, the earlier work failed to foresee the declining power of communicable-disease programs or the economic difficulties that would continue to affect the poor majorities in the developing world.

To take these factors into account, demographers have been revising their mortality as well as their fertility assumptions; and their mortality revisions are as sobering as their fertility revisions are hopeful. In 1968, for example, the United Nations was estimating that life expectancy in the developing world as a whole would increase to 68 by the year 2000, which would be 13 years more than the present 55. The current estimate for the year 2000 is around 62 years. If present trends continue, in other words, the increase between now and the end of the century will be more nearly 7 years than 13 years. The change implies that, in the year 2000, the infant mortality rate in developing countries will be around 65 deaths per 1,000 live births, compared with 35 to 40 deaths if the earlier assumption had prevailed; it suggests 11 million to 12 million infant and child deaths annually in the developing world at that time, down somewhat from the present total of around 15 million, but also about twice the 5 million to 7 million that would have been occurring annually under the earlier assumption.

A slowdown in population growth involving such numbers of infant and child deaths, especially one appearing on the eve of the 1979 International Year of the Child, cannot be considered an unambiguous blessing. The need is not simply for a zero population growth rate, but rather for a zero rate of growth achieved through a population equilibrium at low levels of both mortality and fertility. Judged by this standard, the present slowing in the world's rate of population growth is seriously inadequate.