A Ph.D is a filter in that respect, as you have to work hard to achieve it.
And I there are many hard working shop floor workers, but they never make it to management. Just working hard on it's own does not earn you more money.
I agree entirely. Income increases by the unique value that you can bring to the table, not the degrees you have hanging on the wall.
If everyone in the world studied the exact same subject in college, incomes would remain the same as if those people had not gone to college at all. The market forces would remain unchanged.
What I mean by work is to work to set yourself apart. Maybe you need to hit the gym to become unique that way. Maybe you need to work on your singing skills to become unique that way. College can also bring you unique attributes, but it is no more special than any other avenue, at least with respect to income.
Again, college has nothing to do with it on a macro scale. I won't deny anecdotal instances can use college to their advantage. It definitely happens. Random degree holder is not going to be any better off though.
And I there are many hard working shop floor workers, but they never make it to management. Just working hard on it's own does not earn you more money.