How does Piketty handle this serious problem? [The problem that his proposed tax on capital would hurt labor?] He doesn't. The only behavioral response to a tax on capital that he discusses at length is that owners of capital would move to lower-tax countries. And to avoid that happening, he puts a lot of thought into how to form, essentially, a tax "cartel" in Europe. He would have countries in the European Union agree to tax capital, making it harder for people to move to lower-tax countries.
Even an economist who likes Piketty's book and favors his tax on capital has pointed out its bad effects on economic well-being. In his New Republic review, MIT economist Robert Solow, who won the Nobel Prize in economics for his pioneering work on economic growth, wrote:
The labor share of national income is arithmetically the same thing as the real wage divided by the productivity of labor. Would you rather live in a society in which the real wage was rising rapidly but the labor share was falling (because productivity was increasing even faster), or one in which the real wage was stagnating, along with productivity, so the labor share was unchanging? The first is surely better on narrowly economic grounds: you eat your wage, not your share of national income. But there could be political and social advantages to the second option. If a small class of owners of wealth--and it is small--comes to collect a growing share of the national income, it is likely to dominate the society in other ways as well.
Translation: if capital is taxed heavily, workers' well-being will not improve, but because a tax on capital will likely stem the increase in the share of income going to owners of capital, wealthy people will dominate the society less than otherwise.
For Piketty and, presumably, Solow to calmly countenance the possibility of stagnating real wages just to keep capital's share from increasing, they would have to see some large problems with increasing inequality. Solow does not point out any such problems, which makes sense because his review is short. But Piketty, in over 600 pages, does not make a clear statement about why increasing inequality is a problem in a society where almost everyone's lot in life is getting better and better.
So let's fill in the gaps. How big a problem is wealth inequality? In my opinion, if people came by their money without cheating others and without getting special government favors, then there is no problem with those people becoming very wealthy. What really matters is inequality in consumption and, here, the differences between poorer Americans and wealthier Americans are probably as low as they have ever been. Most lower-income people have color televisions, cell phones, refrigerators, comfortable clothing, and three square meals a day. That was not true 60 years ago. Or take a longer view: In the mid-19th century, the poorest people in American were probably slaves. That was, of course, awful. The largely rich people who "owned" them could treat them very badly if they wanted to. And even if they did not want to, let me repeat that these poor people were slaves.
Or consider finer differences between the middle class and the wealthiest. You would have to look carefully--at least, I would--to see the difference in the quality of clothing that billionaires and those with a net worth of "only" $100,000 wear. Both can travel by jet, but the wealthier person can get there more quickly and easily on his private jet. The rest of us have to share space. The private jet is certainly nicer, but is that really a major social problem?