For someone to not understand this without a study, I think they would need to be oblivious to the state of the economy, the cost of living in the average metropolitan area where the average American lives, and social dynamics / cultural issues in America.
Also, if you've experienced significantly different levels of salary then this is obvious.
"Money doesn't buy happiness" is also considered obvious common sense. The study is looking for the phase boundary where one obvious statement ceases to apply and the other takes over.
Also, if you've experienced significantly different levels of salary then this is obvious.