For the past three years, a bizarre thing has happened in the United States. After rising every year since 1959, life expectancy suddenly started declining in 2014, when it was an all-time high of 78.9 years. Americans now live, on average, 78.6 years on average. A drop of 0.3 years doesn't sound like much—it's not even four months. But the U.S. is alone in this downward trend of life expectancy, and according to the Kaiser Family Foundation, it is also last among a dozen other developed countries.

Life expectancy is calculated using public health records and census data, and it's a useful model for analyzing quality of life. The last time life expectancy dropped in the U.S., 1915 through 1918, the country was beset by both World War I, which killed over 116,000 American soldiers, and the outbreak of the Spanish flu, which killed 675,000 people in the U.S. and an estimated 50 million worldwide.

Death from chronic health conditions, like heart disease and cancer, are not on the rise—in fact they’re in decline in some places. Instead, more and more deaths are a result of misery-driven maladies like drug overdoses, liver disease, and suicide—longer-term trends that have accelerated more rapidly in recent years. The number of overdose-related deaths, for example, increased steadily since 1999 before spiking sharply to 63,632 in 2016 and 70,237 in 2017, according to the Centers for Disease Control. The story is similar for suicide rates, which are currently at their highest levels since World War II. Suicides have increased 33 percent since 1999 and are worst in states with looser gun restrictions. And deaths from chronic liver disease, often the result of alcohol abuse, also rose by 30 percent in the same time period.

These are what clinical researchers have started calling "deaths of despair," the result of misery and hopelessness rather than accident, illness, or war. And they're afflicting mainly working-age people—more and more Americans are dying in the so-called prime of life, between the ages of 25 and 64 in particular, according to a Journal of the American Medical Association study. What's most remarkable is that the U.S. is alone in these trends. Speaking to NBC News, Steven Woolf, the author of the study, pointed out that many of these "deaths of despair" are clustered in depressed regions of the country, like the Rust Belt, which has suffered economically since the U.S. steel industry contracted sharply in the 1970s. Woolf also called the spike in early deaths a "distinctly American phenomenon."

The change in life expectancy is even more surprising considering that the racial gap is at a historic low: 75.54 years for black Americans and 79.12 years for white ones, according to the CDC. Part of that is because the recent opioid epidemic hasn't hit black communities as hard—opioid addiction often begins with legitimate pain-pill prescriptions, and since historically doctors haven't taken black patients' reports of pain as seriously as white patients’, black people were prescribed opioids at far lower rates.

The more extreme gap is between the wealthy and everyone else. An in-depth study from the University of Washington found that life expectancy in the U.S. fluctuates by as much as 20 years depending on where people live, and that difference is driven largely by socioeconomic and "race/ethnicity factors"—something that can't be fully separated from socioeconomic factors in a country as segregated as the U.S. Researchers also determined that this geographic inequality has grown steadily since the 1980s, and it's only likely to increase in the future. "This is way worse than any of us had assumed," one of the study's authors told the Guardian.