The notion of capitalism as a mechanism in which “fat and happy” companies become acquisition targets is changing, writes Jason Zweig in last week’s Wall Street Journal.

Zweig cites new research that shows “U.S. companies are moving toward a winner-take-all system in which giants get stronger, not weaker, as they grow.” A few “superstar firms,” he says, have evolved to dominate and crowd out competitors. Twenty years ago, he writes, the U.S. had more than 7,000 public companies, and today there are fewer than 4,000. Between 1997 and 2014, for example, Zweig says that the real estate sector has seen a decrease from 42 publicly traded companies to 20 (which accounted for 78% of combined revenues). The “winners”, Zweig says, are also capturing most of the profits.

Economist Roni Michaely of Cornell University suggests that technology as well as declining enforcement of antitrust rules might be among the causes for the shift. “If you want to compete with Google or Alphabet,” he argues, “you’ll have to invest not just billions, but tens of billions of dollars.” Michaely and a team of economists have found that if investors bought into industries where “the top companies were growing more dominant, while betting against sectors whose top firms were becoming weaker, you would have outperformed the overall stock market by an average of roughly nine percentage points annually between 2001 and 2014.”

Zweig speculates that a situation where “too much money is chasing too few stocks” might explain the “seemingly unstoppable rise of the stock market.” He adds, however, that “history offers a warning. Many times in the past, winners have taken all—but seldom for long.”