If you ask a liberal politician who their favorite economist is, there are three likely responses. One response is a panicked change of topic. From the slightly more sophisticated politicians who skim the New York Times, you might hear Paul Krugman. From the politicians who style themselves intellectuals of the left, you’d hear Thomas Piketty.

Thomas Piketty is a French economist who is a professor at the School for Advanced Studies in the Social Sciences, one of France’s most prestigious schools. In 2013, he released a book called Capital in the Twenty-First Century. It was a bestseller. In the book, Piketty argues that the rate of return on capital is greater than long-run economic growth. He projects that this relationship will continue and a small minority of capital owners will become richer, not just in absolute terms due to economic growth, but also in relative terms, as wages comprise a smaller and smaller share of the economy over time.

Piketty thinks this is bad for typical left-wing reasons and suggests that massive wealth taxes are an appropriate solution to what he sees as the otherwise inevitable concentration of wealth. The New Yorker has argued that Piketty’s work inspired Senator Elizabeth Warren’s campaign for a wealth tax.

While Piketty’s claims about the desirability of egalitarianism and wealth taxes are to some extent subjective, his book and his reputation are premised on the claim that he carefully assembled data that showed an increasing return to capital over time. He claims to have based his work on three centuries of data.

Unfortunately, for fans of Piketty and left-wing economists who prefer a more fact-based economics approach, it looks like Piketty’s data work is quite sloppy. In 2015, Magness and Murphy pointed to a wide range of flaws or mistakes in the book writing that they found “evidence of pervasive errors of historical fact, opaque methodological choices, and the cherry-picking of sources to construct favorable patterns from ambiguous data. Additional evidence suggests that Piketty used a highly distortive data assumption from the Soviet Union to accentuate one of his main historical claims about global “capitalism” in the twentieth century.” According to Magness and Murphy, Piketty bases his measure of 150 years of the world economy on a sample size of just six individual years and just extrapolates the rest of the data! Yikes!

The critique was influential but perhaps as Alex Tabarrok suggests it was possible to dismiss because the criticism was published in a libertarian-leaning journal. However further work showed even more problems with the Piketty data.

In 2018, a report from the left-wing Urban Institute came out that showed that work by Piketty which showed extreme increases in economic inequality was an outlier in the field with other studies, even some by other left-wing economists, showing much smaller effects. This is consistent with Magness and Murphy’s concern that Piketty was cherry-picking results to conjure up an image of an inegalitarian dystopia. In 2023, another study tried to look at the same topic and pointed out what you accounted for the extremely progressive American income tax system and income transfers (welfare programs), there was “little change in after-tax top income shares.”

Inequality changes over time. Sometimes it rises. Sometimes it falls (as is happening right now). Piketty didn’t discover a secret law of economics and now we have the data to show it. That probably won’t stop some left-wing politicians from citing him- there are even people who cite Karl Marx unironically- but it might let us know who we don’t have to take seriously.