In the case of the Affordable Care Act, private insurers have priced their policies on the premise that a lot of healthy young people will purchase insurance. If the young don't participate in large numbers, and older and sicker people do, then insurers will have to raise premium charges for everyone. That will be blamed, rightly or wrongly, on Obamacare. We know from the administration that more than two million people have become newly insured. That's good news. But we don't know who they are. Is the Affordable Care Act succumbing to the dread dynamics of adverse selection? The early signs are far from conclusive, but they are not encouraging.