I don't know why this is being downmodded. In 1908 the US population was roughly 90 million. Part of the huge economic growth experienced over the 20th century had to do with population growth and consolidation as the world's global superpower. Even though the rest of the world also had economic growth, other economies didn't fare so well over the same period of time.

In a way, looking at the past century and assuming that the next one will be similar in terms of return rates is like expecting Google's stock to grow spectacularly over the next decade. Once you are #1 there is not as much room for explosive growth.