As I write this, America's population reportedly has passed the 300 million mark. The most remarkable aspect of this landmark event is how unremarkable it really is.

"If I had my way, I would build a lethal chamber as big as the Crystal Palace, with a military band playing softly, and a Cinematograph working brightly, and then I'd go out in back streets and main streets and bring them all in, all the sick ... the maimed; I would lead them gently, and they would smile me a weary thanks ..."

That was D.H. Lawrence daydreaming about population control. He was hardly alone. During the so-called Progressive Era, "enlightened" social planners were convinced that overpopulation was the gravest problem facing Western society. That's why Lawrence gave "three cheers for the inventors of poison gas."

George Bernard Shaw, a thoroughgoing eugenicist, believed that the "the majority of men at present in Europe have no business to be alive." H.G. Wells smiled at the prospect that the "swarms of black and brown and dirty-white and yellow people" will "have to go." In America, Wells' onetime girlfriend, Margaret Sanger, the founder of Planned Parenthood, argued that birth control was essential to stem the rising tide of the unfit. Leading feminists, Progressive economists and legal theorists shared a similar vision. Supreme Court Justice Oliver Wendell Holmes, who concluded in the case of Buck v. Bell that the state had the power to forcibly sterilize "defectives," believed that forced population control was at the very heart of Progressive reform.

The Holocaust diminished the popularity of eugenics, but the panic over overpopulation endured. Paul Ehrlich, author of the scaremongering "The Population Bomb," predicted in 1970 that between 1980 and 1989, roughly 4 billion people, including 65 million Americans, would starve or otherwise meet their doom in the "Great Die-Off." Inspired by such fears, Alan Guttmacher, the former president of Planned Parenthood, was a champion of coerced birth control - i.e. "compulsory sterilization and compulsory abortion" - throughout much of the world.

Today, overpopulation anxieties pale by comparison to years past. But simply because people aren't proposing mass murder and forced sterilizations - or predicting that twice the population of California will starve to death in a country where obesity dwarfs hunger as a health concern - hardly means current anxieties are reasonable.

These days, overpopulation is primarily a hang-up for environmentalists, though suburbanites and feminists occasionally whine about it, too. And an important part of the argument has changed. While before, Progressives were worried about the "muck" at the low end of the global population, they're now vexed by the fat cats at the top.

Americans consume more of the earth's resources, they complain, and produce piles more greenhouse gasses. At the environmentalist fringe, there's even a growing movement to convince eco-friendly Americans to voluntarily reduce or eliminate their own reproduction in order to ease the strain on Mother Nature. Since the political orientation of your parents is the single best determinant of your own politics, you can expect a lot fewer environmentalists in a couple decades if this idea catches on.

What unites today's worriers and those of yesteryear is their common allegiance to Malthusianism. The British economist Thomas Malthus argued that population will always outstrip available resources. And he was 100 percent wrong.

Because people are, in the words of Julian Simon, "the ultimate resource." Given the right policies, intellectual and economic productivity trumps biological reproductivity. "Between 1820 and 1992," Ronald Bailey writes in Earth Report 2000, "world population quintupled even as the world's economies grew 40-fold." Productivity matters more than other statistical measures because it demonstrates we're doing more with less. That's why, for example, starvation is a political disaster, not a natural one. There's literally too much food in the world. There's also plenty of land left. You could move the entire world population inside medium-sized homes and they'd all fit inside Texas, yielding a population density similar to that of Paris.

Today's Malthusians still look askance at economic productivity, believing that it's better to limit growth at a "sustainable" rate, which means consigning billions of poor people to lives that threaten the environment (poor people treat their environments like expendable resources rather than priceless luxuries) and, worse, threaten their own lives. It's more enlightened than dreaming of a giant gas chamber, to be sure. But that's got to be small solace for those trapped at the bottom.