The growing importance of the population race will be very hard for any political leader to fully digest. Every nation prefers to think of itself as productive in the sense of hard-working and smart, not just fertile. But population is where the real action is.

Comparing six of the leading developed countries — the United States, Germany, Japan, Canada, Australia and Britain — I found that not only has productivity growth been slowing across the board in recent decades, but also that the gaps in productivity growth among these rich nations are narrowing sharply. For example, in the 1990s and 2000s, productivity was growing much faster in the United States than in Germany or Japan, but that advantage has largely disappeared in this decade.

The reasons for this convergence are complex, possibly having to do with the way production technology now spreads quickly across borders. But this trend spans the developed world, and it basically holds regardless of which two countries you compare, which should raise doubts about how any one country, including the United States, can regain a distinct economic advantage by focusing only on reviving productivity.

Which brings us back to babies and immigrants. Like productivity, population growth has been slowing worldwide in recent decades, the big difference being that the gaps among the rich nations are increasingly significant. In the 1960s the United States population growth rate averaged 1.2 percent, or 50 percent higher than Europe’s and about the same as Japan’s. By the late 1960s, population growth peaked worldwide because of the spread of birth control and other cultural shifts, but it has slowed much more gradually in the United States than in its rivals.

In the past decade, American population growth has averaged 0.8 percent a year, eight times faster than Europe’s, and Japan’s population has not grown at all. Increasingly, then, the underlying difference between the fast- and slow-growing economies is explained more by the differences in population growth than by productivity. And the United States now relies more than ever on demographics to defend its economic power. In the past decade, population growth, including immigration, has accounted for roughly half of the potential economic growth rate in the United States, compared with just one-sixth in Europe, and none in Japan.