It has been a long time since Lewis Townsend, a local obstetrician, has heard a patient express any anxiety over world population. Some patients say they are limiting the size of their families for financial reasons; others, because of their age. But no one talks about doing her part for zero population growth.

"I heard 'zero growth' about 10 or 15 years ago," said Townsend. Now, "it's not a concern."

Two decades ago, overpopulation was widely perceived as an urgent global threat, a steady but volatile force fueling a host of other social, economic and environmental dangers. But today the issue has virtually disappeared from the public agenda, frustrating population specialists and environmentalists who argue that, if anything, the rapidly growing human population poses even more of a threat.

They point to several reasons for the diminished public concern, including a dramatic shift by the federal government in 1985 when the Reagan administration withdrew population-control funding from organizations it deemed had supported abortion in any country. The linkage to abortion has stifled efforts to control population internationally and chilled debate on the issue, these groups say.

Overpopulation also lost its immediacy because of the substantial decline in fertility rates in this country since the late 1960s, when rapid growth was cited frequently as a critical reason for American couples to limit the size of their families.

But while fertility is dropping in this and other developed countries, world population continues a steep expansion. Despite the link between population growth, pollution and resource depletion, environmentalists say the topic has been missing from the recent upsurge in environmental activism in this country.

"Public awareness about population issues has waned dramatically since the '60s," said Susan Weber, executive director of Zero Population Growth. She dated the change in attitude nationally to a CBS newscast in the early 1970s, when anchorman Walter Cronkite reported that fertility rates in this country had fallen below "replacement level."

"That was perceived as the end of our problems," she said. And as attention to the issue began to fade, the political climate of the Reagan and Bush administrations hastened the trend, she said. "All population birth control programs, even the notion that population is a problem," came under attack.

While the United States is still the major contributor to international population control, funding has dropped from more than $298 million a year in 1985 to about $256 million this year, according to the Agency for International Development.

Scientists do not agree on whether a crisis is imminent, but there is a consensus among demographers that rapid population growth, especially in certain "hot spots," is a serious global problem. And few dispute that the number of human beings on the planet is multiplying at breathtaking speed.

While world population in 1960 was just over 3 billion, there will be 3 billion people living in just the five largest countries by the turn of the century, and another 3 billion elsewhere around the globe.

Fertility rates have declined in about half of the Third World and virtually all industrialized countries. Globally, the rate of population growth has begun to decrease, from 2 percent in 1968 to 1.8 percent today.

Still, the current rate of growth means world population will double every 39 years. And the U.N. Population Fund estimated last year that world population -- now 5.3 billion -- would reach 14 billion before it stabilizes late in the next century. An earlier estimate said the stabilization would occur at 10 billion.

Numbers of such magnitude carry little meaning for the average American, however, and scientists hold a range of opinions on what conclusions should be drawn.

Most prominent among those who see disaster in the numbers is Stanford University biologist Paul Ehrlich, who launched the popular movement for zero population growth with his 1968 book, "The Population Bomb." In a new book published this spring with his wife, Anne, Ehrlich renewed his earlier warnings that overcrowding will lead to massive starvation and environmental catastrophe. Decreasing fertility in many industrialized countries should not bring complacency, he said.

"The biggest fallacy is that there is a population problem in Africa and India, but not here," he said. "It's not density, not numbers, it's what people do to the planet."

For example, the United States, with less than 5 percent of the world's population, produces more than 17 percent of the global "greenhouse" gases.

In his new book, "The Population Explosion," Ehrlich argues that couples in the United States should continue to limit their number of children to one, or at most two, because resource consumption is so much higher here.

"The birth of an American child is hundreds of times more of a disaster for Earth's life-support systems than the birth of a baby in a desperately poor nation," he writes.

At the other end of the spectrum are a handful of "pronatalists" who have issued admonitions that falling fertility in industrialized countries will lead to economic decline.

Economist Julian Simon, a professor of business at the University of Maryland, argues that population growth should be viewed as an economic stimulus because more people will mean greater innovation, production and market formation.

While the parameters may be defined by Ehrlich and Simon, the population debate is conducted elsewhere in more subtle tones.

"It's not as simple as either side would have you believe," said Michael Teitelbaum, a demographer at the Alfred P. Sloan Foundation.

"World population," said Teitelbaum, "doesn't mean that you're going to have a catastrophe next year. It doesn't mean you will have a world of catastrophes. But you could have some very serious problems in some countries and regions."

Demographers tend to focus on problems in south Asia and sub-Saharan countries. In Bangladesh, for example, 100 million people live in an area the size of Wisconsin.

Population-issue groups, which point to the political linkage between population control and abortion as a major barrier to addressing the problem spots, had hoped the election of George Bush would mean a change in policy. But Bush has continued to deny funding to organizations even for abortion referrals.

The decline of the U.S. fertility rate had little to do with overpopulation, according to experts.

Arthur Haupt, a population specialist at the Population Reference Bureau, said it is generally assumed that U.S. fertility fell -- from 3.4 children per woman of child-bearing age in the early 1960s to 1.8 in the 1980s and 2 in the past year -- because of the availability of contraceptives, higher educational levels among women, increased participation of women in the work force and other lifestyle changes, such as delayed marriage.

Haupt suggested that the extraordinarily high fertility of the baby boom era may have been a reason for the attention devoted to overpopulation in the late 1960s.

Hundreds of Zero Population Growth chapters formed around the country, and Life magazine ran a cover story on the subject. The attention was heightened by the stark message in Ehrlich's "The Population Bomb," in which he declared that "the battle to feed all of humanity is over."

Ehrlich and others who have issued similar grim warnings have since been criticized as "doomsday" theorists, and lingering skepticism created by that criticism may have further dampened public attention to population concerns.

Simon, for example, labeled the warnings by Ehrlich and others as "nonexistent scares . . . every single prediction of the doomsayers has turned out wrong."

"Not as many died as we expected to die," Ehrlich said. But he defended his statement that "hundreds of millions" would starve, citing World Health Organization and U.N. estimates that between 10 million and 14 million people die each year from hunger and related diseases.

Alan Lopez, a statistician for the World Health Organization, said it is widely accepted that more than 14 million children under age 5 die each year worldwide, but a significant portion of those deaths are unrelated to hunger.

To assume that 10 million of the 14 million deaths are hunger-related -- as Ehrlich has done -- is "taking the upper limit," Lopez said.