A single health insurance company dominates the commercial insurance market in more than 4 in 10 of the nation's metropolitan areas, according to an analysis by the American Medical Association. The study showed that competition among health insurers continues to shrink, despite several proposed mergers falling through earlier this year, including deals between Aetna and Humana, and Anthem and Cigna. Previous studies have shown that consolidation among insurers leads to lower payments for doctors and higher premiums for patients.

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