At Power Line, Steven Hayward is, among other things, Mr. Data - the contributor with the handy chart or visual. Today he has a really amazing New York Times chart based on Federal Reserve data showing the "Expected Wealth Benefit for 4-year College Graduates as Compared with High School Graduates."
I'm in that first tall black column on the left. It is probably true I accumulated something like three times as much wealth as a high school graduate my age. With graduate level education, it may be even more.
Why has it declined over time? Supply and demand - too many graduates chasing too few jobs which require additional schooling. Plus decreased quality - grade inflation and dumbing down the coursework.
And majors matter - degrees in Victims Studies and other non-occupational topics. Finally, as in life, timing is vital.