Guns, drugs and cars contribute substantially to the life-expectancy gap between the United States and other developed nations, a study found.

Deaths from old-age ailments sometimes get more attention in longevity research, but deaths from these three causes tend to happen at younger ages, contributing to many decades of life lost, the researchers said.

Data from the Centers for Disease Control and Prevention show injuries including violence, car crashes and drug poisonings and overdoses are the leading cause of deaths for Americans up to the age of 44.

U.S. death rates from these three injury categories exceed those in 12 other developed countries included in the study: Austria, Denmark, Finland, Germany, Italy, Japan, the Netherlands, Norway, Portugal, Spain, Sweden and the United Kingdom, the researchers said.

Among men, life expectancy in 2012 in those countries was 78.6 years versus 76.4 in the United States. Injury-related deaths accounted for almost half of that difference, the study found.

Among women, life expectancy was similarly higher — 83.4 years versus 81.2 years in the United States. Injury-related deaths accounted for less of the difference — about 20 percent.

Gun deaths were a major factor among men: The U.S. rate was 18.4 such deaths per 100,000 men, versus 1 per 100,000 in the comparison countries.

Among women, drug-related deaths explained most of the injury-related difference: the U.S. rate was 10 per 100,000 women versus fewer than 2 per 100,000 among women in comparison countries.

The researchers' estimates are based on an analysis of 2012 data from the U.S. government and the World Health Organization. The government study was published in the Journal of the American Medical Association.

"If we brought mortality from car crashes, firearm injuries and drug poisonings down to levels that we see in these other countries, we'd gain about a year of life expectancy," said lead author Andrew Fenelon, a sociologist with the CDC's National Center for Health Statistics.

The injury data include accidental and intentional deaths and suicides, and deaths from prescription medications and illicit drugs.

The study bolsters the argument that improving U.S. life expectancy will require addressing premature deaths among younger ages, said Jessica Ho, a Duke University sociologist who has done similar research.

Strengthening U.S. gun laws, making safer cars and addressing the root causes of drug use, including income inequality, are among policies that might help, she said.