Whether the average American can be expected to live a longer or shorter life affects such programs as Social Security, which relies on projections it makes for the future based on such numbers.

The brief compares U.S. life expectancy, separately for men and women, to nine other countries in the Organization for Economic Cooperation and Development (OECD), focusing on those two factors—smoking and obesity—and how they contribute to the poor performance of the U.S.

The statistics indicate that the major source of the U.S. shortfall rests with women, thanks to the rise in women smoking and its delayed effect on their health—a lag of two to three decades, it reports, before the effects of smoking show up in the population.

And even though there’s a decrease in smoking, there’s a rise in obesity and the associated poor nutrition and diseases that come along with it.

Using regression analysis, the study isolates the impact of smoking and obesity on life expectancy, and finds that if U.S. patterns had matched those of its peer countries, U.S. life expectancy would have exceeded the average until very recently.

Cause-of-death statistics, it reports, suggest that diseases associated with smoking and obesity—such as stroke, cardiovascular diseases, diabetes, lung cancer and respiratory diseases—are the major reasons for the U.S. lagging behind its peers.

Despite the drop in smoking, the rise in obesity has taken its toll on the population, and the study concludes that the relative performance of U.S. life expectancy in the future depends on controlling obesity.