If you wanted to pick a single metric to judge the success of a country, one measure that both conservatives and liberals could agree on, life expectancy may be your best bet. On that metric, the U.S., which spends more on health care than any other developed country, is doing terribly.

A new study in the Journal of the American Medical Association shows just how badly. Between 1990 and 2010, the U.S. fell from 20th to 27th place on life expectancy among the 34 countries of the Organisation for Economic Co-operation and Development. U.S. life expectancy at birth rose by only 3 years, to 78, while other countries saw greater increases.

Even more damning is the gender gap behind those numbers. Most of the increase came from men living longer, shrinking the gap between the sexes. Life expectancy for women stagnated: Between 1985 and 2010, women in almost half of U.S. counties showed no improvement at all. (You can find an interactive map with county-level data here.)

What's killing Americans earlier than their foreign counterparts? The JAMA study found the greatest risk factor, as measured by the number of related deaths, was diet, followed by tobacco, high blood pressure and obesity.

There are bright spots. Vastly fewer years of life were lost to HIV/AIDS in 2010 than in 1990; pre-term birth complications and lower respiratory tract infections also fell in the rankings. But the relative cost, measured by years of healthy life lost, increased for drug and alcohol abuse, kidney disease and schizophrenia.

Where does that put the U.S. compared with other countries? For the five leading causes of lost years of life -- heart disease, lung cancer, stroke, chronic obstructive pulmonary disease and road injury -- the U.S. ranks higher than the OECD average on each one but stroke. For two of those, road injuries and COPD, the U.S. ranks 32nd out of 34. (It gets worse. For interpersonal violence and Alzheimer's, the U.S. is 33rd; for poisonings, it's dead last.)

Conservatives may see this data as proof that Americans are just fatter, more violent, more prone to risky behavior and fundamentally less healthy than people in other countries, making meaningful comparisons with those places irrelevant. By that light, complaints that Americans spend more money for worse health outcomes overstate the ability of health-care policy to make a difference -- and, by extension, exaggerate the importance of government action.

By contrast, liberals will probably read these numbers as reinforcing the argument that government matters very much. At a press conference announcing the findings, Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington and the study's lead author, noted that policy changes -- everything from anti-smoking campaigns to more playgrounds -- have made Americans healthier than they would otherwise be.

The challenge is coming up with still better policies to get Americans living as long as people in other countries. Actually, the first challenge is agreeing on that goal in the first place, and then deciding how to attain it.

The inescapable context for this study is the enduring debate over whether the government should ensure that almost all Americans get access to health insurance. The portion of Americans who are uninsured, or who are insured but lack affordable access to care, is the single biggest difference between the U.S. and other developed countries. It's also, through Obamacare, a difference that can be fixed.

Policymakers can't wave a wand and make Americans healthier. They can get a step closer, however, by implementing a law that's already on the books, that pays for itself, and that has a real chance of improving health.

(Christopher Flavelle is a member of Bloomberg View's editorial board. Follow him on Twitter.)