Many insurers only dipped a toe into the Affordable Care Act's online marketplaces for their first year. Cigna, one of the country's largest insurers, offered 2014 plans to individuals in fewer than half a dozen states. Humana is only in a little more than a dozen states. The biggest health insurer, UnitedHealthcare, didn't offer any policies through the federally run online portal and only a few elsewhere. The result: substantially higher premiums than otherwise would have been the case, according to a new paper. Economists Leemore Dafny, Jonathan Gruber and Christopher Ody ran alternate-reality experiments in markets in the 34 states served by healthcare.gov, the federal exchange.