Due to improvements in medicine, sanitation, and other public health advances, life expectancy in most of the developed world has increased nearly every year over the past century. Despite this trend, life expectancy in fallen in the U.S. for the second year in a row.

While Americans enjoyed the highest life expectancy of any OECD nation in the 1960s, the U.S. health advantage began to wane in the 1980s and eventually fell below the OECD average in 1998. The U.S. average life expectancy at birth today is 78.7 years, 1.5 years lower than the OECD average.

Relative to other affluent nations, Americans report worse birth outcomes, more injuries and homicides, higher teenage pregnancy rates, and higher incidences of HIV/AIDS, obesity, diabetes, and heart disease.

Click here to see the states with the longest and shortest life expectancy.

Life expectancy also varies heavily across the United States. While life expectancy in some states is greater than the OECD average, in others it is on par with developing countries like Malaysia, Uruguay, and Iran. Differences in life expectancy throughout the United States largely parallel differences in socioeconomic conditions, like income and education, and risk factors like smoking, inactivity, and obesity.

To determine the states with the longest and shortest life expectancy, 24/7 Wall St. reviewed 2014 life expectancy at birth figures provided by the Institute for Health Metrics and Evaluation, a global research center affiliated with the University of Washington.

Factors that contribute to poorer health among Americans relative to the OECD include unhealthy behaviors such as high caloric intake and firearm ownership, weaker welfare support, lack of universal health insurance, and a physical environment that encourages car ownership over pedestrian and bicycle transportation.

Within the United States, states with the most risk factors have the lowest life expectancy. The eight states with the lowest life expectancy also have the eight highest inactivity and eight highest obesity rates of any state. Other behavioral risk factors, such as smoking, correlate highly with life expectancy. Tobacco use is the leading cause of preventable death in the United States, and life expectancy for smokers is at least 10 years shorter than it is for nonsmokers. In 13 of the 15 states with the shortest life expectancy, the smoking rate is higher than the national figure of 18%.

In recent years, one of the major factors reducing life expectancy in America has been the opioid crisis. Drug addiction has devastated families and ravaged communities. More than 64,000 Americans died from drug overdoses in 2015 — more than the number of fatalities the United States suffered during the Vietnam War. Mississippi and Kentucky, which have the two highest drug overdose mortality rates, were among the five states with the largest declines in life expectancy from 2010 to 2014, and now have the lowest and seventh lowest life expectancy of any state, respectively.

While the relationship between income and health is complicated, wealthier individuals are far more likely to live longer than less wealthy individuals. Higher-income Americans can afford fresh, healthy food, and they have easier access to gyms and other opportunities for physical activity and to preventative medical care. Of the 15 states with the highest life expectancy, the median annual household income is below the $57,617 national median in only one. Similarly, the poverty rate in 13 of the 15 states with the longest life expectancy is well below the 14% national figure.

Education is closely related to income, and as a result is a major determinant of health. In addition to increasing the likelihood that an individual will obtain a high-paying job, a college education can provide one with the tools necessary to make healthier choices about diet and exercise. In 13 of the 15 states with the longest life expectancy, the share of adults with a bachelor’s degree is greater than the 31.3% national share.

To determine the states with the longest and shortest life expectancy, 24/7 Wall St. reviewed 2014 life expectancy at birth figures provided by the Institute for Health Metrics and Evaluation, a global research center affiliated with the University of Washington. Data on smoking and obesity came from County Health Rankings & Roadmaps program, a collaboration between the Robert Wood Johnson Foundation and the University of Wisconsin Population Health Institute. Data on median household income came from the U.S. Census Bureau’s 2016 American Community Survey. All data are for the most recent period available.

Editor’s Note: A previous version of this article incorrectly stated that drug overdose deaths in the U.S. exceeded the number of casualties sustained by Americans during the Vietnam War. In fact, drug deaths in 2015 exceeded total U.S. fatalities during the war.