Varför har inkomstskillnaderna ökat?

Enligt professor Kevin Murphy beror det i hög grad på att avkastningen på utbildning har ökat:

Of course, the other side of the equation is that the return to going to college—that is, the return on your investment, if you invest the time, money and effort to go to college—is higher today than it’s probably been in half a century. That’s a good thing. When we say we have a higher return on investment, whether you earn more on your stock market investment or on your college investment, we think that that’s a good thing. It means there’s greater opportunity out there for individuals and society as a whole to increase our incomes by increasing our investment in people, by investing more in their education.

Så en fråga till dem som beklagar ökade inkomstskillnader är: Spelar det ingen roll hur inkomstskillnader uppkommer för hur man ska utvärdera dem normativt? Är det negativt om inkomstskillnader beror på att utbildning ger högre inkomst?