The 2017 edition of an annual report on commercial health plans by the American Medical Association (AMA), “Competition in Health Insurance: A Comprehensive Study of U.S. Markets,” finds that in 43 percent of metropolitan areas one insurer has at least a 50 percent share in that market. This market concentration is one factor making it difficult for new insurers to enter a market, which reduces competition and tends to lead to higher premiums and lower payment for physician services. To learn more, go to