In itself, that might not be bad. But the authors identified a deeply troubling result of an economy where just a few top-tier companies dominate. One of the economic truths of much of the 20th century was that the portion of the country’s overall income that went to labor was constant; as the economy grew, workers got a proportionate share of that growing pie. But labor’s share of the national income has been shrinking over the past few decades. This is true in many countries, and the decline speeded up in the United States in the 2000s.

The trend puzzles economists. Some suggest it reflects the rise of cheap robots that can do the jobs of human workers, but the data isn’t convincing. Instead, Katz and his coauthors blame the emergence of the superstar companies. As these companies grow and become more efficient and more adept at using digital technologies, they need fewer workers relative to their soaring revenues. The fact that these labor-frugal firms have so much of the market share in their sectors means labor gets a smaller portion of the nation’s overall income.

Compounding the problem is that superstar companies, which desire the best possible talent, tend to pay much better than anyone else. This dynamic is deepening the divide between the country’s economic winners and losers. Nicholas Bloom, an economist at Stanford, and his colleagues have shown that about one-third of the growth in U.S. income inequality since 1980 can be explained by the disparity between the pay premiums of a few elite companies and the salaries most workers earn. Fewer and fewer people—mostly a select group of highly trained professionals—are enjoying the vast profits generated by these top companies. It is “certainly a big part of the [economic] anxiety” that is plaguing the country, Katz believes.

The rise of the superstar companies also might help explain another disturbing economic trend. Despite the proliferation of impressive new advances in software, digital devices, and artificial intelligence over the last decade and the great profits generated by Silicon Valley, economic growth in the United States and other developed countries has been sluggish (see “Dear Silicon Valley: Forget Flying Cars, Give Us Economic Growth”). In particular, an economic measure called total factor productivity, which is meant to reflect innovation, has been dismal (see “Tech Slowdown Threatens the American Dream”). How can overall growth be so lackluster while the high-tech sector is booming?