Wojciech Kopczuk is a Columbia economics professor who co-authored (in 2004) one of the leading estimates of wealth concentration in the literature, along with current progressive darling Emmanuel Saez. But, ever since Thomas Piketty’s bestselling book on wealth inequality, Saez has published research with Gabriel Zucman showing dramatically different results. Kopczuk explains why economists can disagree on the basic facts of wealth concentration, and why there is controversy about Saez and Zucman’s latest claim that billionaires pay a lower income tax rate than the working class.

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