And to throw in my own two cents worth on the subject, I predict that North America is far from immune when it comes to the subject of population growth, even though the U.S. would have been near-zero percent population growth since the 1970's had it not been for immigration. Global warming food security issues due to changing trends in agricultural production, along with our economic debt predicament could lead to huge population migrations to North America. Because our only economic model relies upon growth, political leadership might opt for this easy solution to our debt crisis since growth is all about demographics. I'm still reeling from the census report showing that my state of Colorado has grown by over 18% in the past ten years. Ask Al if that's "sustainable."