The U.S. health insurance industry relies heavily on private markets, but these markets are highly concentrated and are growing more so over time. In a new analysis, researchers document concentration across commercial, Medicare Advantage, and Medicaid markets, examining how asymmetric information—particularly adverse selection—interacts with market power to shape premiums, plan designs, and consumer welfare. They conclude that consolidation of insurers boosts premiums, and they call for regulation and antitrust oversight.