Thanks in large part to immigration.
And of course, a large chunk of the 300 million are people whose parents had large families in the 1950s and who themselves haven't had any children.
The people who whine and cry most about Mexicans "stealing" jobs and the loss of America's "ethnic balance" are often people who haven't replaced themselves.
Replaced themselves.....Why does "Invasion of the Body Snatchers" come to mind.
No, the people who whine and cry most about Mexicans "stealing" jobs are those who haven't tried to find enough "American" workers to pick a whole apple crop recently, and would probably whine about the prices if the farmers did so.
Those who whine and cry most about America's "ethnic balance" are racists.
Having children to try to ensure a color balance in the country, or the world, is probably the worst reason I can think of to do so. The human species will become whatever color it's going to become with or without my input. More power to them!