But let’s put it in Mankiw's inimitable words:
Because capital is subject to diminishing returns, an increase in its supply causes each unit of capital to earn less. And because increased capital raises labor productivity, workers enjoy higher wages. In other words, by saving rather than spending, those who leave an estate to their heirs induce an unintended redistribution of income from other owners of capital toward workers.
The bottom line is that inherited wealth is not an economic threat. Those who have earned extraordinary incomes naturally want to share their good fortune with their descendants. Those of us not lucky enough to be born into one of these families benefit as well, as their accumulation of capital raises our productivity, wages and living standards.Now let’s just make a list of the assumptions you have to make in order to accept Mankiw's argument (none of which he mentions himself):
1. All resources are fully employed, and the economy is on its production possibility frontier.
2. A decision to save, by lowering the cost of capital, increases the quantity of investment.
3. Financial and real capital are identical, and the return to the first is the return to the second.
4. All savings and investment occur in the same economy; rich people do not earn income from investments elsewhere.
5. All prices represent true social costs and benefits. There are no profits to be made except by increasing the net wealth of the community. For instance, transfers and uncompensated externalities play no role whatsoever in profits.
6. There are no monopoly profits, with the exception of self-extinguishing temporary monopolies associated with wealth-creating innovations.
7. But, in partial contradiction to (6), there is no technological change at all, since it would alter the marginal productivities of labor and capital.
8. Production sets are convex everywhere; there are no increasing returns or interactions between resources or activities that would give rise to nonconvexities and multiple equilibria.
On top of all this, it should be pointed out that, if Mankiw is right, the rate of profit—Piketty's r—should fall as the capital-income ratio rises. But a central argument in Piketty's book is that r is remarkably consistent through relative capital accumulations and decumulations, a steady 4-5%. There isn't a single dollop of data in Mankiw's little piece that challenges Piketty's finding.
Putting all of this together, it doesn't sound like the sweeping conclusion at the end of Mankiw's column is justified, does it?
But Greg’s a smart guy! He doesn't really think that his op-ed is summing up the state of scientific knowledge. He knows everything I've written above. Some of it is even in his textbook. Clearly his goal is not to make a defensible economic argument.
To take him seriously is to miss the point. When he wrote this column Greg had a twinkle in his eye. He’s thinking to himself, “This is really going to annoy the liberals!” That’s what the central message of microeconomics is about, after all, once you put aside all the caveats and unlikely assumptions: self-interest is good for everyone, a free market is the optimal form of economic organization, and there is no conflict of interest between the rich and the rest. In real economics these propositions are hypotheses to be examined and quite often rejected, but in ideological economics they are ammunition to attack the left.
When Mankiw teaches Econ 10 at Harvard, he has many kinds of students scattered through the auditorium. Some are bored. Some are intrigued but have lots of questions that intro econ can’t answer. Some walk out. But you can be sure there are a few whose eye’s light up when they hear about the virtues of free markets and self interest. They're the ones who are thinking, “Wow, that must really annoy the liberals!” A significant chunk of them will decide to become econ majors and then go on to get PhD’s and teach their own Econ 10's.
Of course, most economists aren't like this. I think a majority are fairly centrist in their politics and moderately skeptical of ideologies that rest on a raft of assumptions and a paucity of data. But every econ department at a college or university seems to have at least one of the Mankiw spawn, whose greatest pleasure is piss off the do-gooders. When they populate recruitment committees and journal editorial boards the twinkle becomes a scowl, and ideological rigidity becomes a filter for the rest of the profession.
It’s not all in fun.