So, how can a growing economy accelerate? Imagine a factory owner who wants to expand his shoe-manufacturing capacity. The owner can invest in shoelace machines and employee training to increase the per-worker productivity of the factory. He could also simply hire more workers. Just like that hypothetical factory owner, the economy’s growth fundamentally comes from just two things—productivity growth and labor-force growth.

The trouble with rapid productivity growth is that it’s a bit like permanent happiness—much easier to obsess over than to achieve. Indeed, economists obsess over productivity quite a bit, but they often disagree about what increases it, and, as some of them sheepishly admit, they’re not even all that great at measuring it. So far this century, productivity growth in the U.S. has been consistently low—even negative—since the end of the dot-com bubble. Several studies suggest that as rich countries like the U.S. get older, their productivity-growth rates naturally decline. (Since there’s a lot about productivity that puzzles economists, they aren’t entirely sure why this happens, either.) Designing a budget projection around a sudden surge in productivity is a bit like betting one’s life savings on the discovery of alien life on the moon. Not utterly hopeless. But certainly not advisable.

So, what’s the trick to raising GDP if productivity levels are subdued? More workers.

In the second half of the 20th century, economic growth in the U.S. rode a labor-force boom, after the Greatest Generation gave birth to the Boomers, then the largest generation in history. But in the last decade, that demographic wind has turned against the U.S.—and most advanced economies. As the Obama White House said in its 2013 economic report, “real GDP in the United States is likely to be permanently slower than it was in earlier eras because of a slowdown in labor force growth.” It’s not just that population growth is slowing down. What’s more, the share of Americans between 25 and 54 who are working—a statistic known as the “prime age labor participation rate”—has been generally declining since the late 1990s.

There are two simple ways to add more people to the U.S. population: more babies and more immigrants. The trouble with increasing fertility is that no advanced economy seems to have figured out how to do it. The U.S. is in the middle of a protracted lull in baby-making—but so is Scandinavia, and Western Europe, and Japan, and Russia. Low birth rates may simply be a consequence of gender equality and overall prosperity: As a nation’s share of educated women grows, its fertility rate tends to decline, perhaps because working women don’t have the time, money, or interest in raising the sort of large families that were so common (and necessary) in agrarian economies.