TWO beliefs continue to shape debate on Obamacare. First, pre-existing medical conditions shouldn’t prevent people from obtaining affordable health insurance. And second, people who don’t want health insurance shouldn’t be forced by the government to purchase it.

These may seem to be reasonable positions. But they are incompatible. That’s been shown by historical events, and it’s now being strikingly confirmed by recent experience in the emerging Obamacare insurance exchanges.

The crux of the matter is what economists call the adverse-selection problem. Uninsured people with pre-existing conditions often face tens or even hundreds of thousands of dollars in out-of-pocket medical costs annually. If insurers charged everyone the same rate, buying coverage would be far more attractive financially for people with chronic illnesses than for healthy people. And as healthy policyholders began dropping out of the insured pool, it would become increasingly composed of sick people, forcing insurers to raise their rates.

But higher rates make insurance even less attractive for healthy people, causing even more of them to drop out. Before long, coverage would become too expensive for almost everyone.