Remember the 2014 bestseller “Capital in the Twenty-First Century” by French economist Thomas Piketty? Beachgoers in the Hamptons couldn’t be seen without it tucked under an arm if they wanted to be regarded as serious people concerned about the plight of the less fortunate. The dismal tale of exploding inequality and capitalist failure has been a recurring theme in political chatter ever since. But a new report highlights just how poorly Mr. Piketty’s thesis has held up under further study.

This column should note that some...