The great bulk of the economic commentary you read in the papers is focused on the short run: the effects of the “fiscal cliff” on U.S. recovery, the stresses on the euro, Japan’s latest attempt to break out of deflation. This focus is understandable, since one global depression can ruin your whole day. But our current travails will eventually end. What do we know about the prospects for long-run prosperity?

The answer is: less than we think.

The long-term projections produced by official agencies, like the Congressional Budget Office, generally make two big assumptions. One is that economic growth over the next few decades will resemble growth over the past few decades. In particular, productivity — the key driver of growth — is projected to rise at a rate not too different from its average growth since the 1970s. On the other side, however, these projections generally assume that income inequality, which soared over the past three decades, will increase only modestly looking forward.

It’s not hard to understand why agencies make these assumptions. Given how little we know about long-run growth, simply assuming that the future will resemble the past is a natural guess. On the other hand, if income inequality continues to soar, we’re looking at a dystopian, class-warfare future — not the kind of thing government agencies want to contemplate.

Yet this conventional wisdom is very likely to be wrong on one or both dimensions.

Recently, Robert Gordon of Northwestern University created a stir by arguing that economic growth is likely to slow sharply — indeed, that the age of growth that began in the 18th century may well be drawing to an end.