I'm very well aware of what people's argument is for why the US became dominant, and I'm sure it was a factor. Yet that didn't happen to other regionally isola

I'm very well aware of what people's argument is for why the US became dominant, and I'm sure it was a factor. Yet that didn't happen to other regionally isolated countries like Mexico or Brazil - hell, or even Canada. And yes, youth always volunteer for wars. Like I said, I'm not a historian and my example was just an example of how things could guide selection. That said I don't think many countries had something similar to what ronin, or even samurai, were. It was a fairly unique system. China's Great Leap Forward would be another example of something that could guide who lived to procreate and who didn't.