**SNIP** To function as intended, the marketplaces need a broad, healthy risk pool to keep staggering rate increases from occurring. The premiums of healthy, cheap-to-insure people cover the big bills for the relatively small number of sick people. So if the exchanges do not “enroll enough young, healthy people, insurers will have to raise everyone’s premiums,” wrote the New Republic’s Jonathan Cohn in May. “In the worst case, this could create what actuaries call a ‘death spiral’: Rising premiums prompt people to drop out, causing premiums to increase even more.” Because the reform was aimed at bringing coverage to most...