So what's the impact of a rising population?
Greater demand for capital overall to maintain a consistent amount of capital per effective worker (so America's hungry demand for foreign capital--and the resulting deficits-- make sense here).
Necessity for increased capital could also prove to be the mother of greater productivity invention.
Europe and Japan: consistent amount of capital per effective worker, but flat growth unless added productivity makes each worker... well... more productive.
America's population | Now we are 300,000,000 | Economist.com