Also, people are staying in education for longer and deferring earnings. A 23 year old working at Walmart will probably out earn a 23 year old studying for their masters while they are both 23 but probably not when they are both 30.
Some of them, but retail management salaries aren't very high unless you are running the entire store. In many cases supervisors aren't paid much more than the staff they manage and they still want graduates for higher management positions.
It is very easy to become a store manager (refrain from screwing up and take transfers when available for promotions), and those jobs pay exceptionally well. A Walmart or Winn-Dixie store manager is going to be making six figures, and at least in the case of Winn-Dixie, they actively avoid people with advanced degrees because district managers are afraid of getting passed over.
I don't see how this is a factor when they compare a change in difference between college graduates and not over time. College graduate in 1989 would also be deferring their earnings.
