Barack Obama’s signature health-care law is struggling for one overriding reason: Selling mispriced insurance is a precarious business model.

Aetna Inc. dealt the Affordable Care Act a severe setback by announcing Monday it would drastically reduce its participation in its insurance exchanges. Its reason: The company was attracting much sicker patients than expected. Indeed, all five of the largest national insurers say they are losing money on their ACA policies and three, including Aetna, are pulling back from the exchanges as a result.

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