For more than three decades, Chinese women have been subjected to their country's brutal one-child policy. Those who try to have more children have been subjected to fines and forced abortions. Their houses have been razed and their husbands fired from their jobs. As a result, Chinese women have a fertility rate of 1.54. Here in America, white, college-educated women—a good proxy for the middle class—have a fertility rate of 1.6. America has its very own one-child policy. And we have chosen it for ourselves.

Forget the debt ceiling. Forget the fiscal cliff, the sequestration cliff and the entitlement cliff. Those are all just symptoms. What America really faces is a demographic cliff: The root cause of most of our problems is our declining fertility rate.

The fertility rate is the number of children an average woman bears over the course of her life. The replacement rate is 2.1. If the average woman has more children than that, population grows. Fewer, and it contracts. Today, America's total fertility rate is 1.93, according to the latest figures from the Centers for Disease Control and Prevention; it hasn't been above the replacement rate in a sustained way since the early 1970s.

The nation's falling fertility rate underlies many of our most difficult problems. Once a country's fertility rate falls consistently below replacement, its age profile begins to shift. You get more old people than young people. And eventually, as the bloated cohort of old people dies off, population begins to contract. This dual problem—a population that is disproportionately old and shrinking overall—has enormous economic, political and cultural consequences.

For two generations we've been lectured about the dangers of overpopulation. But the conventional wisdom on this issue is wrong, twice. First, global population growth is slowing to a halt and will begin to shrink within 60 years. Second, as the work of economists Esther Boserups and Julian Simon demonstrated, growing populations lead to increased innovation and conservation. Think about it: Since 1970, commodity prices have continued to fall and America's environment has become much cleaner and more sustainable—even though our population has increased by more than 50%. Human ingenuity, it turns out, is the most precious resource.