One lesson from recent economic troubles has been the usefulness of history. Just as the crisis was unfolding, the Harvard economists Carmen Reinhart and Kenneth Rogoff — who unfortunately became famous for their worst work — published a brilliant book with the sarcastic title “This Time Is Different.” Their point, of course, was that there is a strong family resemblance among crises. Indeed, historical parallels — not just to the 1930s, but to Japan in the 1990s, Britain in the 1920s, and more — have been vital guides to the present.

Yet economies do change over time, and sometimes in fundamental ways. So what’s really different about America in the 21st century?

The most significant answer, I’d suggest, is the growing importance of monopoly rents: profits that don’t represent returns on investment, but instead reflect the value of market dominance. Sometimes that dominance seems deserved, sometimes not; but, either way, the growing importance of rents is producing a new disconnect between profits and production and may be a factor prolonging the slump.

To see what I’m talking about, consider the differences between the iconic companies of two different eras: General Motors in the 1950s and 1960s, and Apple today.