Post-graduation education would certainly be subject to the upper-bound effect. But aren’t there a very small absolute number of people who advance beyond the 4-yr degree — so small as to be insignificant compared to what is happening to the “feedstock cohort”?

I’ve seen some research, that isn’t at my fingertips, indicating that the majority of students would be better-served by some practical education, e.g.rather than by liberal arts 4-yr degree programs. There’s empirical evidence that there’s a good bit of “qualification inflation” for entry jobs due to an excess of grads with non-useful training.

They mention a reduction in the rate of increase in years of education, but how much of this change is simply an approach to some limit? In other words, a kind of market “saturation” in which once everyone is going to university, there’s no room for increase.

Then there’s another threshold (with its own set of barriers - competitive admissions, cost, deferred earning years) between the four-year university programs and graduate or professional programs: law school, med school, MBA programs, etc. But a lot fewer people do that than a four-year Bachelors program.

Goldin and Katz do mention this effect “In 1980, each year of college raised a person’s wage by 7.6 percent. In 2005, each year of college yielded an additional 12.9 percent. The rate of return from each year of graduate school has risen even more — from 7.3 to 14.2 percent.” Though to me this looks like marginal advantage and makes going to grad school look like not such a great deal (now they tell me).

As to the “network” effect: it would be difficult to get some handle on that but you could break down the analysis to compare highly-competitive top-tier schools (Ivy Leagues, Stanford, Duke, MIT, CalTech etc.) to other institutions.

There’s another variable to consider if trying to analyse a “network” effect: Stanford and MIT have highly competitive admissions to begin with.