Despite spending more on health care than any other country in the world, the U.S. is lagging in life expectancy. American men and women do not live as long as people in France, Japan, and other wealthy nations, and the gap is widening. Now, the National Research Council has attempted to find out why that is. Here, a quick guide to their findings:

How much does our life expectancy lag?

We rank 36th in the world, behind even South Korea and Cuba, with an average life expectancy of 78.3 years. Japan, whose citizens can expect to live to the ripe old age of 83, ranks first. But what is especially worrying is how we're failing to keep up with other developed nations. Between 1980 and 2006, the life expectancy for men born in the U.S. rose by only 5.5 years — a lower rate than in 21 other developed countries. Women didn't fare much better, gaining six years in life expectancy since the 1950s, while women from nine other high-income countries gained eight years.

What did the National Research Council say was behind the gap?

Smoking, primarily. Even though only about 21 percent of U.S. adults smoke these days, reports Katherine Hobson in The Wall Street Journal, that number was closer to 37 percent back in 1980. "Smoking-related illnesses such as lung cancer can take decades to have an impact on mortality rates." Men will likely see their average longevity increase in the next few years, as their "smoking habits are well past the peak," says NPR. Women, however, "took longer to cut back, and the toll of smoking will be around longer."

Are there other factors?

Obesity also plays some part, but just how much is a "controversial" question, says The Boston Globe. The NRC estimates that obesity could "account for up to a third of the shortfall compared with other rich nations." Lack of exercise is also a factor. Americans are "among the most sedentary people" in the developed world, says Nathan Seppa at Science News, "vying with Poland for the dubious status of topping that category."

Sources: National Research Council, WSJ, NPR, Boston Globe, Science News, Reuters