The Times reported Oct. 30 that Democratic candidate Jay Inslee had to backtrack on his claim that preventive care could save state government hundreds of millions of dollars a year. It turned out that the state already is doing several of the things Inslee proposed (promoting generic drugs, membership in Group Health) and that the Democrats' gubernatorial candidate had been using an outdated estimate of how much King County had saved in its preventive programs.

Exaggeration is common in politics. So is belief, and in this case belief may be more important. Most Americans believe preventive medicine cuts costs, and they are mostly wrong.

Before the last presidential election, in 2008, the New England Journal of Medicine ran an article summing up all the studies of preventive medicine. The article, “Does Preventive Care Save Money? Health Economics and the Presidential Candidates,” quoted candidates of both parties spouting this belief: Hillary Clinton, Barack Obama, John Edwards and Mike Huckabee. We’re hearing it again from Obama this year.

In a review of 279 preventive programs or procedures, only 20 percent saved money. The rest may have saved lives or otherwise improved health, but didn’t save money.

This is counterintuitive. Consider colonoscopy. If you have a colonoscopy, and it detects precancerous problems, it might prevent a hospital stay for cancer. That saves money for your insurer. But the question was not just about you; it was about the procedure, and to judge that, you have to include the costs of colonoscopies on patients in which no problem was detected.

That doesn’t mean you shouldn’t get a colonoscopy. If your doctor recommends it, get one. Your interest is your life, not saving money. But let’s not pretend in our political debates that it saves money if it doesn’t.

Some things do save money. An effective anti-smoking program saves money. Ditto weight loss. Childhood immunizations. Some other things. King County has done some smart things, and has reduced the rate of growth in its health care costs. In that sense it has saved money. It has not, however, spent less money than the year before.

Preventive care is a perfect idea for politicians: they can offer you more stuff under the label of costing less. It's free. Another such perfect idea is “cover everyone, and it will cost less.” That should mean that covering the uncovered is also free, but it doesn't mean that. It means that if you rope in healthy people and make them pay more than their share, it might lower costs for the already covered.

Very rarely can you save money by giving more people more stuff. Once in a while, the magic works. Mostly it doesn’t.