Gains in life expectancy for U.S. citizens continue to lag behind those in other countries. Perhaps surprisingly, the usual suspects—obesity, smoking and other factors—are not the cause, according to a new study.

The 15-year survival rate for Americans aged 45 to 65 has steadily improved since 1975—but at a slower pace than many other industrialized nations, according to researchers at Columbia University’s Mailman School of Public Health. For their study, researchers compared the U.S. to Australia, Austria, Belgium, Canada, France, Germany, Italy, Japan, the Netherlands, Sweden, Switzerland, and the United Kingdom.

The slower rate of improvement in life expectancy has taken the U.S. from the top of the list in 1975 to the bottom of the list in 2005. Researchers note that factors such as smoking and obesity are not the culprits in this situation. America’s lag in life expectancy rates is likely the result of healthcare system itself, researchers say. Unregulated fee-for-service payments and a reliance on specialty care drive healthcare costs up without comparable gains in life expectancy. The study appears online in the journal Health Affairs.