NEW YORK -- Nancy Strumwasser, a high school teacher from Mountain Lakes, New Jersey, always thought she'd have two children. But the layoffs that swept over the U.S. economy around the time her son was born six years ago helped change her mind. Though she and her husband, a market researcher, managed to keep their jobs, she fears they won't be so fortunate next time.
"After we had a kid in 2009, I thought, 'This is not happening again,' " says Strumwasser, 41, adding, "I never really felt comfortable about jobs, how solid they can be."
The financial crisis that followed the collapse of U.S. investment bank Lehman Brothers in 2008 did more than wipe out billions in wealth and millions of jobs. It also sent birth rates tumbling around the world as couples found themselves too short of money or too fearful about their finances to have children. Six years later, birth rates haven't bounced back.
For those who fear an overcrowded planet, this is good news. For the economy, not so good.
We tend to think economic growth comes from working harder and smarter. But economists attribute up to a third of it to more people joining the workforce each year than leaving it. The result is more producing, earning and spending.
Now this secret fuel of the economy, rarely missing and little noticed, is running out.
"For the first time since World War II, we're no longer getting a tailwind," says Russ Koesterich, chief investment strategist at BlackRock, the world's largest money manager. "You're going to create fewer jobs. ... All else equal, wage growth will be slower."
Births are falling in China, Japan, the United States, Germany, Italy and nearly all other European countries. Studies have shown that births drop when unemployment rises, such as during the Great Depression of the 1930s. Birth rates have fallen the most in some regions that were hardest hit by the financial crisis.
In the United States, three-quarters of people surveyed by Gallup last year said the main reason couples weren't having more children was a lack of money or fear of the economy.
The trend emerges as a key gauge of future economic health -- the growth in the pool of potential workers, ages 20-64 -- is signaling trouble ahead. This labor pool had expanded for decades, thanks to the vast generation of baby boomers. Now the boomers are retiring, and there are barely enough new workers to replace them, let alone add to their numbers.
Growth in the working-age population has halted in developed countries overall. Even in France and the United Kingdom, with relatively healthy birth rates, growth in the labor pool has slowed dramatically. In Japan, Germany and Italy, the labor pool is shrinking.
"It's like health -- you only realize it exists until you don't have it," says Alejandro Macarron Larumbe, managing director of Demographic Renaissance, a think tank in Madrid.
The drop in birth rates is rooted in the 1960s, when many women entered the workforce for the first time and couples decided to have smaller families. Births did begin rising in many countries in the new millennium. But then the financial crisis struck. Stocks and home values plummeted, blowing a hole in household finances, and tens of millions of people lost jobs. Many couples delayed having children or decided to have none at all.
Couples in the world's five biggest developed economies -- the United States, Japan, Germany, France and the United Kingdom -- had 350,000 fewer babies in 2012 than in 2008, a drop of nearly 5 percent. The United Nations forecasts that women in those countries will have an average 1.7 children in their lifetimes. Demographers say the fertility rate needs to reach 2.1 just to replace people dying and keep populations constant.
The effects on economies, personal wealth and living standards are far reaching:
Births might pick up again, of course. In France, where the government provides big subsidies and tax breaks for children, birth rates are back where they were in the early 1970s. In other countries, women who put off having children in the recession might play catch up soon, as they did after World War II. Demographers note that women were having children later in life even before the crisis, and so births are likely to rise anyway.
But even a snapback in births to pre-recession levels will leave families much smaller than they were decades ago, a shift that has already affected industries and economies around the world.
In Japan, sales of adult diapers will exceed sales of baby diapers this year, according to Euromonitor International, a marketing research firm. In Germany and Italy, towns are emptying as families shrink and there aren't enough children to replace older ones who are dying. And in South Korea, where births have fallen 11 percent in a decade, 121 primary schools had no new students last year, according to Yonhap, the country's government-backed news agency.
In China, where the working-age population is set to shrink next year, the government is relaxing a policy that had limited many families to one child. It might not help much. Chinese are choosing to stick to one on their own.
Lei Qiang, a logistics manager in Shanghai with a 2-year-old daughter, has ruled out another child. "I just couldn't think how expensive it is to have two," says Lei, 39.
Economists are worried not just because growth is stalling in working-age populations. Their numbers as a share of the total population in many countries is falling. Economists like to see this share of total population rise, because it means more people are earning money, expanding the tax base and paying for schools for the young and pensions and health care for the old.
Before the recession, the number of these potential workers as a proportion of total population was falling in three of the world's six biggest developed economies -- Japan, Germany and Italy. Now the proportion is also dropping in the United States, France and the United Kingdom, according to investment firm Research Affiliates.
A country can compensate for this demographic drag on economic growth by encouraging people to work longer or to use technologies to increase output. But most economists doubt that such changes are forthcoming or would be enough.
"You need incredible productivity growth," says Michael Feroli, a JPMorgan economist. He says economic growth of 3 percent is unlikely on a "sustained basis" even for the United States, which is blessed with a flow of immigrants, albeit a slowing one, to soften the blow.