Ezra Klein explains:
[T]he public plan will pay prices equivalent to those of private insurers and may save a bit of money on administrative efficiencies. But because the public option is, well, public, it won't want to do the unpopular things that insurers do to save money, like manage care or aggressively review treatments. It also, presumably, won't try to drive out the sick or the unhealthy. That means the public option will spend more, and could, over time, develop a reputation as a good home for bad health risks, which would mean its average premium will increase because its average member will cost more. The public option will be a good deal for these relatively sick people, but the presence of sick people will make it look like a bad deal to everyone else, which could in turn make it a bad deal for everyone else.I agree with every word of that.
The nightmare scenario, then, is that private insurers cotton onto this and accelerate the process, implicitly or explicitly guiding bad risks to the public option. In theory, the exchanges are risk-adjusted, and the public option will be given more money if it ends up with bad risks, but it's hard to say how that will function in practice.
This also illuminates one of the more problematic inconsistencies in the health-care debate. Insurers have been blamed for, among other things, doing too much to discriminate against bad health-care risks and refusing to pay for care far too often. They've been blamed, in other words, for saying "no." But they've also been blamed for doing too little to control costs.
But that is how they control costs. We saw this in the late-'90s, when tightly managed care brought cost growth down to the 4 percent range but also triggered a public backlash (it did not, however, appear to hurt health outcomes).