The US is the third most populous country in the world, with the 2010 census placing its total number of inhabitants just short of 309 million, approximately 4.5% of the total world population.
This is well short of the world's most populous nations, the People's Republic of China and India, which respectively account for some 19.5% and 17.5% of global numbers.
But the US far outperforms any Western nation, except arguably Canada, in terms of population growth. In the second half of the 20th Century, US population increased by 85%, around four times the growth rate in the UK (20%), Germany (20%) and Italy (22%).
Although it took nearly two centuries to reach the 200 million population mark in 1966, the US rapidly gained another 100 million inhabitants, passing 300 million in 2006.
Recent projections suggest that its population growth is slowing but is still well ahead of Europe's.
Having expanded by 13.2% in the 1990s, growth is likely to be around 9% for the first decade of the 21st Century, the lowest since the Depression decade of the 1930s.
An abrupt decline in births and illegal immigration since the economic meltdown of 2008 lies behind the current slowdown.
Nevertheless, the US growth rate over the past decade still exceeds that of China (6%), Britain and France (5%), Japan (near zero) and Germany (which experienced a small decline).
The chief factor accounting for faster US population growth is immigration.
The Immigration and Nationality Act of 1965, which swept away the restrictive immigration laws of the 1920s, ended the so-called "immigrant pause" that began in World War I. The number of first-generation immigrants in the population consequently quadrupled from 9.6 million in 1970 to around 38 million in 2007.
America's new openness, the continuing opportunities it offered for economic gain, and the economic woes of Latin America in the late 20th century, produced a population inflow well in excess of that entering Western Europe.
The immigrants were predominantly Latinos, South Asians, East Asians, and people from post-Communist East Europe.
By contrast, political upheavals, economic disparities among EU member nations, and relative lack of openness and opportunity made Europe less attractive to newcomers.
The new US immigrants also had a significant effect in sustaining fertility rates because immigrant women tended to have more children and at an earlier age.
As a consequence, the total fertility rate in 2010 is projected at 2.06 children per woman, whereas the overall EU rate was barely 1.5.
Two decades of this low fertility rate in Europe, which will determine the future number of child-bearing adults, could produce a decline in EU population from 375 million to around 285 million over the course of this century.
The declining fertility rate is particularly evident in the case of Germany, but is most widespread in rural areas throughout Europe, which once produced large families but have suffered from economic dislocation and outward migration.
The two main EU exceptions to this trend are France and the UK. In combination with increased immigration, the increased fertility rate looks set to bring about an increase in UK population from 61.4 million in 2010 to 70.6 million by 2030, a figure that some analysts consider socially and economically unsustainable.
The immigrant inflow to the US has also compensated for the ageing of the native population once the post-war baby boom bulge of 1945-65 receded.
In 1950, the American age-structure showed width at the bottom among the under-30 bands and was narrowest in the oldest bands.
In 2000, the middle bands (35-54 years) were the widest, as the baby-boom bulge worked its way through the age structure. By 2050, the older bands (55 years and above) will have widened significantly, but will be supported by still wider pre-30 age bands.
This will keep the US median age around 35-38 years at mid-century, but the most pessimistic estimates suggest that the EU median will exceed 52 years (compared to 37.7 years in 2003).
This could result by mid-century in a doubling of the ratio of retirees to workers that would have serious consequences for economic productivity, pension benefits and public programmes.
Official projections suggest a slowing of US population growth to reach 420 million by 2050.
According to a 2008 US Census Bureau analysis, the non-Latino white population, then 68% of the total population, will cease to form the majority in 2042.
The same report projected Latino growth from 15 to 30% of the population, African-American growth from 12 to 15% and Asian-American growth from 5 to 9% by 2050 (making 54% overall).
Pointing to this development, America's two largest states - California and Texas - became "majority-minority" states (with an overall minority population outnumbering the white majority) in 1998 and 2004 respectively.
The political consequences of population trends are intriguing.
The states of the south and west (many of them identified as politically so-called Red states) are growing faster than those of the north and east (mainly Blue states).
This development has produced a steady redistribution of electoral college votes and congressional seats in favour of the former.
Following the 2010 census, for example, New York and Ohio are to lose two House seats each (paradoxically, California will not gain a seat for the first time since acquiring statehood because of the dampening effect of the recession on its population growth).
This may benefit the Republicans initially, but for how long remains to be seen.
As the non-white population grows, the advantage could shift decisively to the Democrats, even in states today considered Red.
If this were the case, the presently rock-ribbed Republican state of Texas may be the first to feel the effects of demographic change on its partisan leaning.