Aon-Willis Merger Would Dominate Outsourced Investment Services

The proposed $30 billion merger of insurance brokerage giants Aon Plc and Willis Towers Watson Plc would create the world’s largest outsourced investment management firm.

Critics worry the giant companies taking over outsourced responsibilities are too big to effectively manage individual investors and that many employers are too small to adequately keep those firms in check. The U.S. Justice Department’s antitrust division filed a lawsuit last week to block the tie-up, saying it wanted to preserve competition among the brokerages that benefits Americans.

The two brokerages are global leaders in the emerging market for outsourced chief investment officers, or OCIOs, ...