1. From a Francophone blogger.

comparing the mean wealth of the x% through time is an implicit selection process where you only select the winners and forget the losers.

In other words, there are two ways to explain why the mean wealth of the x% has grown faster than the mean wealth of the whole population. According to Piketty, it means that the richer you are in the first place, the faster your capital grows over time (hence, the dynastic wealth world he foresees). But it might also be the opposite: this phenomenon is exactly what we should expect to see in a world of high wealth turnover, a world where fortune rewards skills, hard work and risk taking. Quite symptomatically, Piketty and its numerous followers have completely dismissed that possibility.

Pointer from Tyler Cowen.

The phrase “the income of the top X percent grew by z percent” is always a mis-statement, because of turnover among the top x percent. The point made above is that such a figure is always an upward-biased estimate of the actual growth of the actual incomes of actual people in the top x percent.

2. From Martin Feldstein.

his thesis rests on a false theory of how wealth evolves in a market economy, a
flawed interpretation of U.S. income-tax data, and a misunderstanding of the current nature of household
wealth.

These two criticisms cast aspersions on Piketty’s empirical analysis, which Feldstein’s former student Larry Summers said deserves a Nobel Prize.