Mergers are one big reason. Another is the power of so-called network effects — in which the growth of, say, Facebook makes more people want to use it. True, a few industries have become less concentrated, but they are exceptions. If anything, the chart here understates consolidation, because it doesn’t yet cover energy, telecommunications and some other areas. It also doesn’t cover local monopolies, such as hospitals that are dominant enough to drive up prices.

The new corporate behemoths have been very good for their executives and largest shareholders — and bad for almost everyone else. Sooner or later, the companies tend to raise prices. They hold down wages, because where else are workers going to go? They use their resources to sway government policy. Many of our economic ills — like income stagnation and a decline in entrepreneurship — stem partly from corporate gigantism.

So what are we going to do about it? It’s time for another political movement, one that borrows from the Boston Tea Partiers, Jefferson, T.R. and the other defenders of the economic little guy.