You forgot the most obvious one: 3. Those who are more able in the workplace also have improved chances of completing higher levels of schooling thanks to thei

You forgot the most obvious one: 3. Those who are more able in the workplace also have improved chances of completing higher levels of schooling thanks to their ability (or lack of disability). I believe this one is validated by the fact that even as more and more people attain a university education, incomes have held stagnant.