In this case, I dismiss what he says because he's making exactly zero attempt to control for exogenous variables. There's a lot more going on than just changing from a WASP-only aristocracy to a meritocracy of sorts. Vast cultural shifts have necessitated the change, and cause other things to happen as well. Just to pick a few examples, birth rates have dropped, the US population grows only through immigration, basically. Cell phones happened. That changes a lot. Did the WASP-only aristocracy have computers on their desks? No? Did they have The Internet?