Among the mysteries about the incoming administration, Donald J. Trump’s approach to antitrust laws stands out for its importance. Mr. Trump gained a lot of voter support because of the stagnation of working-class living standards in the face of record corporate profits.

Sensing this, he channeled populist anger against elite corporations by, for example, calling the proposed merger between AT&T and Time Warner “too much concentration of power in the hands of too few.”

But the real challenge to competitive markets today does not come from mergers like this one. The great, but mostly unknown, antitrust story of our time is the astonishing rise of the institutional investor — a large company, like a mutual fund company, insurance company, pension fund or asset management firm, that buys stock in substantial quantities for the benefit of clients and customers — and the challenge that it poses to market competition.

In 1950, institutional investors owned about 7 percent of the United States stock market; today they own almost 70 percent. If you count them as a single investor, BlackRock, Vanguard and State Street are the largest owner of 88 percent of the companies in the Standard & Poor’s 500. Control of the economy has not been this concentrated since the Gilded Age. This growing power has undercut middle-class living standards. But there is a way to stop it.