It blew my mind when I truly realized that a couple having two kids does not constitute a net increase in human population. What makes this amazing is wondering where all of these couples having 3-or-more kids are.
Assuming they maintain the birthrate they've had for the past 100+ years (and there is no reason to suspect otherwise), in 2150, the Amish run America[1].
What happens to global politics when a culture that eschews violence gains control of the world's largest nuclear arsenal? What happens to transnational industry and innovation when a culture that balks at accepting the latest technological trends dominates the public arena? Additionally, the 21st Century may be the high-point of multiculturalism in the US. In 2150, over half the population of the USA may be of Swiss-German descent.