Previous posts have discussed changes in the number of births. Jonathan V. Last writes at The Wall Street Journal:
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.
Low-fertility societies don't innovate because their incentives for consumption tilt overwhelmingly toward health care. They don't invest aggressively because, with the average age skewing higher, capital shifts to preserving and extending life and then begins drawing down. They cannot sustain social-security programs because they don't have enough workers to pay for the retirees. They cannot project power because they lack the money to pay for defense and the military-age manpower to serve in their armed forces.