Yet despite the care the administration took in establishing incentives and safeguards, even some of Obamacare's most committed backers are wondering whether the experiment will work as advertised -; or, like Harvard's P.P.O., go off the rails along the way. Adverse selection is perhaps the direst threat. ... Healthier Americans will probably flock to cheaper bronze plans. And insurers will vie to enroll the healthy. In some states, big insurers have chosen not to participate in exchanges to avoid their strictures. On the outside, they could still sell cheap plans to skim off the healthy and avoid a rule that insurers on the exchanges must also offer more generous silver and gold plans (Eduardo Porter, 8/7).