By BERNARD CONDONNEW YORK (AP) a¿¿ 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 2008 financial crisis 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.