While you might think older people are the ones who have to worry most about when and how they're going to die, a big part of why men in various countries have very different life expectancies can actually be attributed to causes of death that affect people under 50.

Case in point: males in the US experience lower life expectancy at birth than men in many other high-income countries, such as the UK and Japan, and according to a new study, effectively half of the reason for that gap is because of just three types of fatal injuries: firearm-related injuries, drug poisonings, and motor vehicle traffic crashes.

Researchers at the National Centre for Health Statistics (NCHS) and Johns Hopkins University examined data from the US National Vital Statistics System and the World Health Organisation Mortality Database. From this, they calculated the death rates by age, sex, and cause for the US and 12 other, similarly developed high-income countries, including: Austria, Denmark, Finland, Germany, Italy, Japan, the Netherlands, Norway, Portugal, Spain, Sweden, and the UK.

Comparing the data, they found that men in the comparison countries enjoyed a 2.2-year life expectancy advantage over their counterparts in the US - 78.6 years versus 76.4 years.

It's a significant gap, but what's even more remarkable is that, according to the stats, the three largest injury-related causes of death in the US – guns, drugs, and cars – account for 48 percent of this gap, shaving some 1.02 years off the life expectancy of the average American male.

"Part of the reason there is such a large difference is that people who die from these injuries usually have several decades left to live," one of the researchers, Andrew Fenelon from the NCHS, told Reuters.

Of that 48 percent, gun deaths make up 21 percent, while drug poisonings and vehicle deaths account for 14 and 13 percent, respectively.

American women on average also experience a shorter life expectancy compared to the comparison countries (83.4 years to 81.2 years), but guns, drugs, and vehicle crashes aren't quite as significant as causes of death, accounting for 19 percent (0.42 years) of the gap. Of this, 9 percent are drug poisonings, 6 percent are motor vehicle traffic crashes, and 4 percent are firearm-related injuries.

"Although the reasons for the gap in life expectancy at birth between the United States and comparable countries are complex, a substantial portion of this gap reflects just three causes of injury," the authors write in The Journal of the American Medical Association.

It's a sobering study, but the bright side of the research is all these causes of death – which cost more than 100,000 lives in the US every year – are dangers we can ultimately do something about.

"All three major problems we know are modifiable, and targeted well-informed effective policies can make a great difference," researcher Benedikt Fischer from Simon Fraser University in Canada, who was not involved in the new study, told Reuters. "This should ring all available alarm bells. If places like Japan or Europe or Canada manage to handle these problems, why wouldn't the US be able to do that?"