Too Many Toys? Mattel-Hasbro Fate Rides on Antitrust Math

A proposed merger of Hasbro Inc. and Mattel Inc., an entity that could account for close to half the toys sold in U.S. mass-market outlets, would need to win approval from antitrust officials in Washington who are increasingly saying no to deals marrying the dominant players in an industry.

Competition watchdogs, who are grappling with a logjam of mergers following a frenzy of deal-making last year, have taken an aggressive stance against tie-ups in concentrated industries, opposing transactions such as Comcast Corp.'s bid for Time Warner Cable Inc. and the merger of American Airlines and US Airways.

An antitrust review ...