Amid the din of daily political combat, it’s easy to overlook long-term trends reshaping the country. The stock market has quadrupled since 2009’s Great Recession low, but the growth of real wages has failed to accelerate, even as unemployment has fallen from 10% to under 4%. An International Monetary Fund paper published earlier this month suggests these developments are linked to a single cause—increasing corporate concentration throughout the U.S. economy.
A standard definition of market power, say the authors of the IMF...