BlackRock, Vanguard and other big institutional investors own roughly 70 percent of the public stock market, according to some reports. People are starting to ask whether this allows companies — now having the same owners — to compete less and raise prices.

In Senate testimony last month, the Justice Department’s antitrust chief, William J. Baer, said the department was investigating the potential antitrust implications of the rise of institutional shareholders. More specifically, he noted that institutional shareholders hold big minority stakes in many companies in the same industry.

Take, for example, the airline industry. Across all of its funds, BlackRock held 8.3 percent of United, 6.6 percent of JetBlue, 4.7 percent of Delta and 4.5 percent of Southwest, as of March 31, 2013. Mr. Baer said the Justice Department was exploring whether such cross-ownership either explicitly or implicitly pushed companies to compete less.

After all, if there is less competition in the airline industry and higher fares result, it will arguably benefit BlackRock over all, because it holds shares in not just one company but several in the industry. Less competition may mean higher costs for consumers but more profits for the airlines and BlackRock’s investors.