PERSPECTIVES ON THE U.N. POPULATION CONFERENCE : . . . It’s Voodoo Science : Somalia has no system for registering births, yet the World Bank lists its population down to a 2% margin of error.

We all know that chemistry grew out of alchemy, just as astronomy developed from astrology and medicine from witchcraft. What, one wonders, will grow out of “population science?”

The question is not flip, nor the analogy irrelevant as the International Conference on Population and Environment opens in Cairo today. The conference’s U.N. managers, like the Clinton Administration and private-sector population-controllers who dominated pre-Cairo U.S. policy-planning, will insist that their prescription for radically expanding coercive state power in the name of “population stability” and “sustainable development” is scientifically grounded. But given what “population science” does not know about the present and cannot tell us about the future, doubts on this score are not merely obscurantist. Consider:

* “Population science” cannot predict the growth rate of human populations over long periods. Because there is no scientific method for predicting birth or death rates with precision, past projections often look ludicrous (one U.N. study famously misprojected India’s population by 100 million). But population projections may become even more difficult in the future. Public-health improvements can change mortality rates rapidly, even if countries remain poor. Birth rates can drop quickly and sharply, even absent anti-natalist governmental policies (as in Japan between 1948 and 1958).

* “Population science” cannot tell us when (much less how) fertility rates will decline, not least because serious scholarship has determined that the relationship between lower fertility rates and economic, social and cultural conditions is extraordinarily complex. Does fertility decrease as income increases? In some cases, yes. But then why does Tadjikistan have both a birth rate and a per-capita output rate twice that of Sri Lanka? Yet another myth--that high levels of health correlate with low fertility--is contradicted by the fact that life expectancy in Kenya today (with a fertility rate of 6.5 births per woman) is about the same as in Germany in the 1920s (fertility rate: 2.3).

* The pretense that Harvard demographer Nick Eberstadt calls “false precision” helps explain why projections in population studies are so difficult. According to Eberstadt, only one-tenth of the Third World’s population is covered by reliable registration systems for vital statistics. Somalia, for example, never had a national census until 1985 and has no system for registering births. Yet the World Bank’s prestigious World Development Report blithely claims a 2% margin of error for its Somali population and birth-rate statistics. Given such hard realities, as Eberstadt notes, numbers like these are simply “guesses dignified with decimal points.”

* Finally, “population science” has no scientifically precise definition of “overpopulation”--a shibboleth sure to be invoked repeatedly at Cairo. How would we know when a society was “overpopulated?” When its “natural increase” (birth rate minus death rate) was unusually high? Then the United States between 1790 and 1800 had serious overpopulation problems. What about high birth rates as a measure of “overpopulation?” That won’t work, either: the U.S. birthrate in the 1790s was 25 points higher than the latest World Bank estimates for India, Indonesia and the Philippines.

Does “population density”--the ratio of people to land--define “overpopulation” precisely? By that measure, and using 1991 U.N. figures, France is more overpopulated than Indonesia, Japan is much more overpopulated than India and Singapore (whose government is trying to raise its birth rate) is far more overpopulated than Bangladesh. Then what about the “dependency ratio”: the proportion of people under 15 and over 65 to the “working age” population? On that index, Ireland and Nepal are about equally overpopulated and the least overpopulated places on Earth are Hong Kong and Singapore.

Can “overpopulation” be determined by rates of emigration? Then Mozambique, Angola and Cuba are overpopulated today, as East Germany would have been before the Berlin Wall went up in 1961.

The images connoted by “overpopulated”--disease, hunger, overcrowding--are not phantoms. But they describe realities that are more accurately denoted as poverty and material deprivation. The Cairo conference might have helped alleviate those problems and the human suffering they cause. It seems unlikely to do so--it may even make matters worse--because its planners are in thrall to “population science.”

Which suggests that the policies proposed in its name should be treated precisely as we would treat the fantasies of any contemporary astrologer or alchemist.