The biggest single contributor to this occurred after WW2, when the majority of white* veterans were able to attend college under the GI bill. The amount of col

The biggest single contributor to this occurred after WW2, when the majority of white* veterans were able to attend college under the GI bill. The amount of college-educated people in the middle classes spiked, employers' expectations raised, and it set up a loop that tightened the association between going to college and finding employment (in certain fields). I'm glossing over a lot of history here (the GI bill had several tremendous effects on postwar America; suburbanization is another huge one), but this was one of the main times when what it meant to go to college shifted. * Though veterans of any race theoretically had tuition paid at any college, they were still mostly segregated.