Forty years ago, "The Population Bomb" by biological scientist Paul Ehrlich was published.

At the time, the book caused a considerable stir with its warning that overpopulation could put the future of civilization in grave doubt.

Lately, though, a "baby gap," a "birth dearth," a "population bust" have been popular topics. That's primarily an issue for Western Europe and Japan, prosperous regions where women don't have enough babies to maintain population in the long run. Russia, with widespread health problems and alcoholism, and relatively low life expectancy, finds its population shrinking by 700,000 people each year. Germany's population is falling about 100,000 a year.

In fact, the birth rate in 25 developing countries stands at or below the replacement level. Around the world, birthrates have plunged from 6 per mother in 1972 to about 2.9 today. Mr. Ehrlich pins this downward trend on government-­sponsored education (especially of women), new job opportunities for women, new availability of contraceptive information and material, and the higher economic cost of large families.

Some economists suggest a declining population can lead to labor shortages, a weaker tax base, fewer customers for business, and a shaky pension system.

To Ehrlich, though, such problems are far less serious than those rising from the current population "bomb." He would like to see the world's population drop voluntarily over many decades to about 1.5 billion to 2 billion, a number that he reckons is sustainable. It would allow people both to live in big cities with their cultural advantages or with nature, as they prefer.

Ehrlich's "Bomb" book led some conservatives to worry about an increased government role in limiting family size. A survey of 15 conservative scholars in 2005 by Human Events magazine ranked his book No. 11 on a list of the "most harmful books of the 19th and 20th centuries." No. 1 was "The Communist Manifesto" by Karl Marx and Friedrich Engels. Others in the Top 10 included Hitler's "Mein Kampf," "Quotations From Chairman Mao Tse-tung," and Alfred Kinsey's "The Kinsey Report."

Today, with 2 million copies of his book sold, Ehrlich remains somewhat puzzled why some conservatives would see his book as so damaging since it didn't advocate a drastic government role. "I don't get it," he says in an interview.

At the time of publication of the "Bomb," the number of people on Earth was about 3.5 billion people. Today there are nearly twice as many, 6.7 billion, and headed toward 9 billion by 2050, perhaps more after that.

Already, says Ehrlich, "we have certainly passed the earth's long-term carrying capacity." Global warming is often regarded as one result of too many people. Water shortages are building not only in California, but also in many nations abroad. Of immediate concern is evidence of a food crisis in the world. Food shortages and high food prices have led to deadly riots in Haiti and civil unrest in Egypt.

Ehrlich estimates that 800 million to 1 billion people today don't have a proper supply of food, many suffering from undernourishment, malnourishment, or worse. Famine threatens, and the world continues to add 75 million a year to its total population, mostly in Africa, the Middle East, and Asia.

Last Friday, more than 140 countries observed World Population Day in various ways to highlight the role of family planning in reducing poverty, promoting development, and saving the lives of mothers and newborns. Thoraya Obaid, executive director of the United Nations Population Fund (UNFPA), stated: "When a woman can plan her family, she can plan the rest of her life."

A group of nongovernmental organizations were to meet in Washington to mark the day. But a high-level representative of the Bush administration was not expected. Both Ronald Reagan and George W. Bush imposed what opponents dub the "global gag rule." This is a policy that bars US family planning aid to private organizations that so much as support the concept of legal abortion. Further, the US under Bush refuses to provide any money to the UNFPA on the basis that such funds might be used, say, for abortion in China.

Ehrlich sees a "lack of leadership" in Washington on the population issue, including both from the White House and from those on the campaign trail.

Family planning, as Ehrlich sees it, leads to both fewer abortions of unwanted children and fewer deaths of mothers lacking access to relatively safe abortions.

In his new book, "The Dominant Animal: Human Evolution and the Environment," Ehrlich and his wife, Anne (who actually helped write the "Bomb"), argue that people are creating a world that threatens "our own species." Many people don't understand the power of exponential growth, he says.