The big health insurer Aetna Inc. announced late Monday it will largely withdraw from state exchanges set up under the 2010 health care overhaul, citing financial losses attributed in part to a controversial premium stabilization program the law established.

Aetna’s exodus from 11 state-based exchanges comes after a more dramatic 30-state withdrawal by UnitedHealth, the nation’s largest health insurer. The departures will reduce the market competition that authors of the health law envisioned to keep premiums low and care affordable.

The decisions underscore how volatile the individual insurance market remains, three years after the exchanges were launched. Major health plans including Anthem Inc. and Humana Inc. have said that they, too, expect losses but have not said they plan to withdraw. Other insurers such as Cigna Corp. and Medicaid-focused plans like Molina Healthcare and Centene Corp. have said they are profiting from that part of their business. Earlier this year, even Aetna said it saw participation as a good investment.

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“We will continue to evaluate our participation in individual public exchanges while gaining additional insight from the counties where we will maintain our presence, and may expand our footprint in the future should there be meaningful exchange-related policy improvements,” Aetna CEO Mark T. Bertolini said Monday.