2011 marked the announcement that the world's population had reached 7 billion. While the media coverage focused on the implications of the growing population and the resulting strains that this growth can cause, less attention was paid to the fact that the population growth varies greatly from country to country. That is, some countries have experienced major population increases while others are experiencing daily population declines.

When we examine population changes at a national level, we need to recall that a nation's population can increase through internal growth (having more births than deaths) and through external growth (having more immigration than emigration). Internal growth rates are strongly predicted by factors like the country's age distribution, desired family size, life expectancy, the use of contraceptives, and the abortion rate.

Because the internal and external factors vary greatly across countries, there are sharp national differences in projected population growth. So which countries are projected to have the largest population growth? The top countries in order of the expected increase in population between 2010 and 2050 are:

(1) India 467 million projected population increase (2) Nigeria 231 million (3) Pakistan 101 million (4) Tanzania 93 million (5) United States 93 million (6) DR Congo 83 million (7) Ethiopia 62 million (8) Philippines 62 million (9) Uganda 61 million (10) Kenya 56 million

The U.S. stands out in this top 10 list for many reasons including its wealth, age distribution, immigration rates and fertility rates. As you probably noticed, the U.S. is the only wealthy country on this top 10 list and, in fact, is the only wealthy country in the top 25. Regarding the fertility rate, the U.S. is the only country in the top 10 with a total fertility rate (defined the expected number of children born per woman in her child-bearing years) near the replacement rate. The other top 10 countries have much higher total fertility rates, vastly exceeding the global average. The U.S., with a median age of 37 years, is the only country in the top 10 list with a median age of over 30 years. In fact, no other country in the top 25 has a median age over 30. Another key difference between the countries in this top 10 list is immigration. Nearly 40 million Americans were foreign born or about 13% of the American population. No other country in the top 25 has more than 6% of its population foreign born. So the US population is wealthier, older, less fertile and more influenced by immigration than the other countries projected to experience major population growth.

While we've seen that U.S. is an outlier compared to other countries projected to have major population growths, it makes us wonder why the U.S. population is expected to grow much more than other wealthy countries? We can start by recalling that the current American population of around 310 million is the third largest in the world, exceeding only by India and China. That means that a 1% increase in the U.S. population contributes far more to the world population than a 1% increase by Mexico, Turkey or any another OECD country.

Besides having a much larger base, what other factors are important in distinguishing the U.S. population projection from that of other wealthy countries?

The population growth difference isn't due to the life expectancy differences. After all, the U.S. has a life expectancy of around 78 years, near the bottom of the OECD countries. Age distribution plays some role. The U.S. has the 7th youngest population of the OECD countries, averaging about 7 to 8 years younger than Japan, Germany and Italy. This younger population means that America has a higher percent of its population in the child bearing years than most OECD countries. Two other key factors driving the expected growth in the U.S. population are America's comparatively high fertility and immigration rates. The U.S. is one of only five OECD countries with a fertility rate near or above the replacement rate with countries like Japan, Korea, Poland and Italy far below the replacement rate. Additionally, the U.S. immigration rate is above the OECD average.

America is often identified as being different from other countries, sometimes in positive ways and sometimes in negative ways. The population projections point to unique aspects of American demographics. Some readers may see the positive opportunities of a vast new generation, the influx of immigrant passion and a manageable support ratio while others will see the negatives from the viewpoint of Malthusian concerns. I suggest we embrace the positives of America's growing population while spending time to understand and address the issues and challenges that this growth creates.

Follow Howard Steven Friedman on Facebook Notes:The Organisation for Economic Co-operation and Development (OECD) is an international organization comprised of 34 countries, representing many of the wealthier countries in the world. Its members include North American countries (Canada, Mexico and the U.S.), much of Western Europe, as well as New Zealand, Australia, Chile, Israel, Japan and South Korea. Data from the OECD is readily available online and makes for a great data source for cross-country comparisons.

Population projections and total fertility rates for this article came from the World Population Prospects 2010. GDP per capita estimates were from the nominal 2011 estimates produced by the International Monetary Fund.