With health care reform in the news, there's been quite a bit of talk about adverse selection and the degree to which it is actually a problem in health care and health insurance markets. Some people have even gone so far as to question whether significant adverse selection effects exist at all outside of textbooks, since when they look at the marketplace, they have a hard time finding it.
But the thing is, if you go looking for it in the marketplace, you aren't likely to find it. Unless the problem has been largely overcome either through the government's intervention or through the private sector institutions constructed to fix the problem (generally intermediaries who can solve the information problem that generates the market failure), the market will fail to exist at all. So you will either observe a fairly well-functioning market that has overcome the problem, or you won't see a market at all.
So if you want evidence of adverse selection, you should look for the institutions designed to overcome the problem - e.g. used car dealers with the expertise needed to overcome the one-sided information problem on car quality, and then issue quality guarantees (or develop a reputation for quality) acting as intermediaries, that sort of thing - and those types of intermediaries are easy to find. Evidence of the institutions needed to overcome adverse selection - and evidence that the problem exists - aren't hard to find. Furthermore, very often government intervention isn't needed, the market can solve this on its own.
And the market will solve it on its own in the case of health care, but we may not like the solution the market comes up with. First, it violates our sense of equity since the solution will be to prevent people likely to have high health costs from getting insurance (or the price of insurance will be so high that they are effectively excluded). But we will still have to provide for them, we can't just abandon them to suffer when help can be provided. It's one thing if someone cannot sell their car due to market failure, it's quite another if they cannot get the medicine or care they need to maintain their health. So the private sector solution may not be morally acceptable. Second, because we have to provide for the sick in any case, the resources that are devoted to excluding people are wasted resources, all that happens is that the problem is shunted off to a generally more expensive option.
So it's not that the private sector cannot solve this problem at all, that's not why we need government to intervene, it's that the solution the market imposes violates our moral sensibilities and wastes resources that could be used more productively.
Update: Thinking about this a bit more, I don't think I want to stand behind the claim that finding evidence of adverse selection is unlikely, for example the consequences of missing markets may be evident in the data (technically the search is then outside of the marketplace, but I still don't want to push this). And I also overstated the extent to which the private sector can solve the adverse selection problem, there are problems that I think the private sector cannot resolve, problems that require government involvement (e.g. mandates). But I do want to comment on this from Megan McArdle:
Of course, it's also true that the population of the uninsured is correlated with something that's also correlated with good health: being young. But then, this sort of undercuts the adverse selection argument, and also the moral imperative of giving them health insurance. If you could reasonably afford health insurance by dropping down to a lower-priced cell phone plan and cutting back on your bar tab, you are not a national emergency.
But this is, in fact, a good example of market failure and why government intervention is sometimes needed. So long as people know that they can get care for life threatening illnesses, broken bones, that sort of thing, and even for less threatening ailments, they have no incentive to cut back on these other expenditures. They get roughly the same care whether they drop their cell plan and the bar tab or not, so why bother?
That's why the government has to mandate coverage, so they are forced to pay their share. Sure, we can say it's a moral issue, that they shouldn't do this, but if they do it anyway then it's all of us, not the people choosing to forgo insurance, who end up picking up the tab. If we are willing to say "let them suffer for their choices, even die for them" then sure, there's no market failure here, they will know that and get insurance (maybe - it's rational to do so, but will they behave rationally?). But I am going to help if I can when health is significantly threatened (even if just to save them from themselves in some cases) - I think most people would - and that leaves the market failure door wide open (I'm not addressing the presumption that people without insurance have cell phones and bar tabs rather than necessities they can cut out in order to afford insurance - that's not, of course, always true).