Thinkers on the left are obsessed with the idea that the disparity in what people earn is the source of society’s problems. Advocates have embraced the work of French economist Thomas Piketty, who argues that income inequality is the real culprit for America’s ills. In 2016, Bernie Sanders tried to turn that explanation into a political message, claiming that what the country needed to do was fight big-money interests, soak the rich, and pursue policies of mass redistribution. Donald Trump’s victory did nothing to dissuade Sanders or the likeminded that income inequality is the crux of the matter.