(FORTUNE Magazine) – LOOKING FOR A NEW REASON TO FEEL PROUD OF YOUR country? Here's one you probably hadn't thought of: America's role in the world economy is about to start diminishing.

We always knew this day would come--the day the American economy as a proportion of the world economy would stop increasing and start declining. While we obviously won't be able to identify that day precisely until well after it has passed, we can be quite confident that it's just about now. In fact, we may have passed it already.

Here's how we know: The U.S. economy's share of world GDP has been increasing for decades. Economy wonks differ on how best to measure these things, but in recent years the U.S. share has been hovering around 31%. Will that proportion get bigger or smaller in coming years? The answer is conceptually simple: If the U.S. economy grows faster than the rest of the world's economies, its share will get larger. But in the reverse case it will get smaller--and the evidence for the reverse case is overwhelming.

Mainstream economists are remarkably united in predicting that our growth rate is going to slow down, and the further out you look, the slower we're going to grow. Our annual growth rate averaged 3.4% over the past 75 years, but from now to 2012 it will average only 3%, says the Bureau of Labor Statistics. Look out a bit further, at the 2007--15 period, and the annual rate falls to 2.9%, says the Congressional Budget Office. Look out a lot further, at the 2004--80 period, and the Social Security trustees assume just 1.8% annual economic growth.

You don't have to be much of a forecaster to predict confidently that the world economy will grow faster than that. It grew about 5% last year, and that figure includes the very large but slower-growing U.S. economy, so the non-U.S. portion was growing even faster--that is, our proportion of the total was falling. A 5% global growth rate may not be sustainable, so let's assume, as most forecasters do, that the long-term rate will be more like 3%. As long as that beats our growth rate by even a small fraction of a percent, our share of the world economy will shrink.

In truth it's easy to see the long-term trend without worrying about exact growth-rate forecasts. Economic growth is an uncomplicated idea: Just think of it as labor-force growth times productivity growth. As America's baby-boomers retire, our labor-force growth is going to slow way down; by contrast, labor force growth in the world's emerging economies is exploding. Our productivity growth is tough to predict, but it certainly isn't going to match the rate in those emerging economies, which are starting from low bases of terrible productivity. So on both factors that determine economic growth, countries that have most of the world's people are going to outdo us.

Why should we be proud? For a reason that may be hard to remember as we go through a transition that many people will find humbling: This is what we wanted. Not only did we want it, but as a nation we worked hard for 50 years to make it happen. After all, a big reason we could be the planet's utterly dominant economy was that after World War II so much of the globe was under the rule of dictators, and one thing we've learned for certain is that dictators equal lousy economies. As long as the Soviet Union, Eastern Europe, China, much of Southeast Asia, much of Africa, and much of Latin America were dictatorships, they were guaranteed to go nowhere economically. Decades of U.S. policy under both parties were dedicated to showing them there is a better way. Finally, almost everyone has gotten the message.

That is what we wanted, and we were right to want it. It's good not just for billions of people around the world but also for Americans. A world with vastly more productive capacity makes America far better off than it would have been if all those countries had continued down the path of North Korea and Cuba. Our smaller piece of the new economic pie will be bigger than any piece we stood to get of the old one.

It's still impossible not to feel just a little sad about our changing place in the world. So remember: We did the right thing. The world is better off, and so are we. And that's worth being proud of.

GEOFFREY COLVIN, senior editor at large of FORTUNE, can be reached at gcolvin@fortunemail.com. Watch him on Wall $treet Week With FORTUNE, weekends on PBS.