Sev has a few theories on this. One of the theories I've heard elsewhere is boomers are the first generation of r select whites, is all previous generations of American whites were k in terms of breeding.

That's not how r vs K work. r vs K selection mostly comes down to number of offspring and time invested raising them to suitable self sufficiency. K has fewer offspring and puts in more time per offspring, which is in no small part due to our own biology where our nascent stage is helpless. Meanwhile r simply plays the numbers game and throws as many offspring as possible out into the world and hopes enough of them last long enough to reproduce. Family sizes have been going down and down over the last century and a bit where if anything it would be trending away from r and towards K. We don't need a dozen children now because we expect half of them to die before hitting double digits age wise.

Regardless of that humans have always been K selection because that's just how our biology works.