ACROSS THE GLOBE, fertility rates are falling. Most industrialized countries are already below the replacement rate of 2.1 children born to the average woman; many more will fall below that crucial mark in the next 25 years. By 2080, world population will probably have peaked around nine billion, after which it will sharply contract (see "The Population Sink).

Why are people having fewer babies with every passing year? The answer is complicated.

Between 1990 and 2000, every region in the world saw the total fertility rate decline. Among nations with rates above the "replacement rate," only two countries saw a rise in fertility: Suriname and Israel, whose rates increased by 0.17 and 0.01 children per woman, respectively. We see the same trend in nearly every country of every size in every climate and with every conceivable political, religious, and economic system--which suggests a complicated set of factors at work. It raises at least the possibility that the root of the problem may involve modernity itself. But for our purposes, let's look at why America's fertility rate has fallen from just over 7.0 to 2.09 over the last 200 years.

IN THE EMPTY CRADLE, his indispensable book about falling birthrates, Phillip Longman postulates a number of forces influencing American birthrates.

Some are obvious: the spread of abortion, contraception, divorce, and women's work opportunities. Another factor is the decline of religious belief (or at least practice) in America over the last 200 years. As Longman writes, 47 percent of those who attend church weekly

There are a host of other small, hidden influences. The social acceptance of homosexuality surely plays some part. And Americans have become more geographically mobile over the years. People now settle farther from their families than ever before - which cuts off a traditional source of support for day-care costs: grandparents. For a variety of reasons, American women have also been putting off childbirth until later in life. Longman notes that "recent studies show that a woman's fertility begins to drop at age 27, and by age 30 can decline by as much as 50 percent." And for practical reasons, the chances of having multiple children decline with age.

Amid the clutter of small influences, two gigantic ones loom: the evolving costs of children and the welfare state.

From the founding of America--indeed, throughout most of history--children have been an economic boon to parents. Children consumed few resources, other than food and clothing, and from a very young age contributed to the household economy by working. Children were once a prime source of nearly-free labor.

By the end of World War II, the expectations for children were beginning to change, and within a few generations, children disappeared almost entirely from the workforce. Today, children are not expected to labor for the family business--they are expected to go to school, and eventually college. Part of this change stems from evolving views of the sanctity of childhood; part of it stems from simple necessity. Small hands, so helpful during the agrarian and industrial ages, are useless in the information age.