Intelligence, education, and more sorting
I would suggest that as America has become more technologically advanced in the last century, higher levels of education became the new basic minimum. In a (comparatively) low-tech society, say a Minnesota farm 100 years ago, a grade-school education was enough, and maybe more than enough, for a youngster to be a successful farmer or farmwife. Almost all the skills needed could be learned on the farm, at Dad's or Mom's knee -- or Grandpa's or Grandma's, for that matter. I would be willing to bet that family and community ties were in general closer than they are now, and that rural people were generally more conservative and more religious than today. This for the simple reason that people had to cooperate to survive, and had to depend on God to keep sane. There were no other alternatives.
The graph here shows that "in the first half of the century, the high-school diploma became the norm," but also shows that in 1910, for instance, less than 10% of Americans had a high-school diploma.
Here is a bit of my family's history that illustrates this very well. My Grandpop Gosslin (born 1887) never finished high school; Grandmom did, in 1905, but she was the only one of her siblings to do so. My parents both graduated from high school in 1931, and they were both college material, but for them college was out of the question because of the Great Depression.
All six of their children (born 1942, 1944, 1947, 1948, 1951, and 1964) finished high school and went to college. Five of us have BA's, four have master's degrees. My three sisters all have master's degrees, and made professional use of them. My older brother's two daughters (born 1969 and 1977) both have doctorates, are successful professional women, and likely will go far, because they're still young and have a lot of time in front of them.
I think this family history illustrates another point too, shown in the top diagram. There are four steps on the ladder. Being on one rung is necessary to get to the next one (and some professions like medicine, law, and engineering have "glass ceilings"), but no guarantee that a person will. A person also needs perseverance, lots of hard work (or cheating), and lots of luck. (In the spring of 1970, Frank "Doc" Whiting, practically the founder of the University of Minnesota Theatre, told us new BA's and MA's that very likely only about 5% of us would ever make a full-time living in the professional or academic theater.)
Last note for this post: to get to "the top," a person sometimes has to step on the faces of others. Doing that -- or even being willing to do that -- requires a lack of conscience, a lack of sense of responsibility to or for others, and a talent for shmoozing, lying, cheating, and being an outright phony. And, I suggest, that's what can (in general) be expected of a generation that hit the college campuses in 1964 and were taught "if it feels good do it," "you can do whatever you want as long as you don't hurt anyone else" (the second part easily forgotten), and "this generation has nothing to learn from the past" (attributed to Margaret Mead).