Including these factors, Winship notes, means that incomes below the top 10 percent have not stagnated but have risen significantly since the 1970s. Increasing inequality is compatible with increases in ordinary people's incomes.
Economist Tyler Cowen takes issue with another of Piketty's assumptions, that the rich can earn 4 to 5 percent on their wealth "automatically, with the mere passage of time, rather than as the result of strategic risk taking."
The French economist, Cowen says, has "a notion of capital as a growing, homogeneous blob" when in fact "sudden reversals and retrenchments are inevitable."
Piketty concedes this is true for people with ordinary incomes. He opposes personal investment accounts in Social Security because there is too much risk of making bad investments.
His assumption that wealthy investors face no similar risks may have seemed plausible in the generation after World War II, when the Fortune 500 list of major companies remained remarkably stable.
But it has made little sense in recent years, when General Motors has gone bankrupt and Google, founded in 1998, is one of the world's most highly valued companies.
"There's a persistent tension," writes Bloomberg's Clive Crook, "between the limits of the data (Piketty) presents and the grandiosity of the conclusions he draws."
Like global warming alarmists, he extrapolates from abstract theory and a few years' trendlines out a century forward -- and presents the results as inevitable.
He also presents them as justifying the confiscation, more or less, of wealth accumulated by private individuals and putting it in the hands of mandarins guided by their supposedly superior sensitivity to public welfare.
There might be less inequality in such a world, but also less economic growth and a lower, though more equal, standard of living.
"In perhaps the most revealing line of the book," Cowen writes, "the 42-year-old Piketty writes that since the age of 25, he has not left Paris, 'except for brief trips.'"
France, where a cozy elite runs government and large corporations, has a 75 percent top income tax rate and essentially zero economic growth. Is that the future American liberals want?