PEOPLE WHO WORRY about population growth and development have long wrestled with a seemingly insoluble problem: if birth rates only fall as living standards rise (as theory and experience in the developed countries suggest), and if developmental projects to raise living standards are overwhelmed by population increases, how are the developing countries ever to break out of the cycle of poverty? Egypt's experience with the Aswan Dam illustrates the difficulty beautifully.The huge dam was planned to irrigate enough additional farmland to feed 4 million Egyptians. While it was being built, Egypt's population grew by 10 million.

Recently published results of the World Fertility Survey indicate that there may be an encouraging answer -- that extreme poverty can also lead people to wish to limit family size. The survey, begun in 1972, is developing detailed information on marriage, fertility and contraceptive practice in 60 nations around the world. The results from the first 15 countries surveyed -- all of them developing countries in Asia and Latin America -- show that fertility has dropped dramatically in all but three. Importantly, in 10 of these countries, more than half of the women who could have more children don't want more.But only half of this group are using effective methods of family planning. Overall, nevertheless, fertility has declined faster than had been thought possible. One expert has called the results evidence of a "demographic revolution." Adding to the impact of the survey's results was the announcement this past weekend that China -- with one quarter of the world's population -- has committed itself to achieving zero population growth between 1985 and 2000.

The reasons for these changes are not yet known. Lengthy analysis of all 60 surveys will be necessary first, but some important conclusions emerge already. One is that government-supported (though voluntary) family planning programs do work -- sometimes extraordinarily well. Another is that the key factor in determining whether women will use contraception is not cultural or social background, as has often been claimed, but simple availability of contraceptives. In Nepal, for example, which has no family planning program and the highest fertility level of those countries surveyed, only 2 percent of married women know of a source of contraceptives within two hours of their home. But if only those few women who live near a family planning facility are considered, the fertility level becomes comparable with that in other countries.

The drop in fertility comes none too soon. World population today grows at a rate of 190,000 people per day, almost 70 million additional people per year. Evidence from a growing number of sources suggests that the ability of the world's biological systems to support them may already be overstrained.Per-capita production from the world's forests, fisheries and grasslands (wood, fish, beef, mutton and wool), for example, have all peaked within the last two decades and are now declining. What the survey's results show is that given sufficient official priority there is a way out -- short of planetary standing room only.