Question

In the united states, which is likely to cause virtually all population growth in the next few decades

Answer 1

Positive net migration. I believe positive net migration will make it happen, from reports there have been millions of illegal immigrants and they havent stopped coming in, like wise we have millions too coming in legally from part of the world into the united states where as only few American go out of their country, mostly just go for tourist and return back to their residents.