How can this paradox be resolved? Well, suppose that those high corporate profits don’t represent returns on investment, but instead mainly reflect growing monopoly power. In that case many corporations would be in the position I just described: able to milk their businesses for cash, but with little reason to spend money on expanding capacity or improving service. The result would be what we see: an economy with high profits but low investment, even in the face of very low interest rates and high stock prices.

And such an economy wouldn’t just be one in which workers don’t share the benefits of rising productivity; it would also tend to have trouble achieving or sustaining full employment. Why? Because when investment is weak despite low interest rates, the Federal Reserve will too often find its efforts to fight recessions coming up short. So lack of competition can contribute to “secular stagnation” — that awkwardly-named but serious condition in which an economy tends to be depressed much or even most of the time, feeling prosperous only when spending is boosted by unsustainable asset or credit bubbles. If that sounds to you like the story of the U.S. economy since the 1990s, join the club.

There are, then, good reasons to believe that reduced competition and increased monopoly power are very bad for the economy. But do we have direct evidence that such a decline in competition has actually happened? Yes, say a number of recent studies, including one just released by the White House. For example, in many industries the combined market share of the top four firms, a traditional measure used in many antitrust studies, has gone up over time.

The obvious next question is why competition has declined. The answer can be summed up in two words: Ronald Reagan.

For Reagan didn’t just cut taxes and deregulate banks; his administration also turned sharply away from the longstanding U.S. tradition of reining in companies that become too dominant in their industries. A new doctrine, emphasizing the supposed efficiency gains from corporate consolidation, led to what those who have studied the issue often describe as the virtual end of antitrust enforcement.