Unequal Income Distribution Hinders Growth?

Nicholas Kristof's recent OpEd piece notes that

"In his important new book, “The Darwin Economy,” Robert H. Frank of Cornell University cites a study showing that among 65 industrial nations, the more unequal ones experience slower growth on average. Likewise, individual countries grow more rapidly in periods when incomes are more equal, and slow down when incomes are skewed.

That’s certainly true of the United States. We enjoyed considerable equality from the 1940s through the 1970s, and growth was strong. Since then inequality has surged, and growth has slowed."