"Half a million people are dead who should not be dead,” Angus Deaton, the 2015 Nobel laureate in economics told my colleagues Lenny Bernstein and Joel Achenbach.

Deaton, an economics professor at Princeton, was talking about the stunning finding that he and his wife, Anne Case, made while analyzing U.S. death data from the past few decades. The researchers found that the mortality rate for white men and women ages 45 to 54 with less than a college education took a sharp turn upward in 1999 — a disconcerting reversal that has been virtually unheard of in advanced countries.

To put that number into perspective, Deaton estimates that a half-million deaths is "about 40 times the Ebola stats." He added,  "You’re getting up there with HIV-AIDS."

The couple's study, which was published in the Proceedings of the National Academy of Sciences this week, offered a number of possible reasons that this might be happening. And since its publication, other experts have weighed in on other trends that might explain this phenomenon. Below is a look at five of their theories. Many of them have to do more with psychological distress than traditional causes of death such as heart disease or lung cancer.

1. The nation's opioid epidemic. "Deaths from drug overdoses among people aged 45 through 64 increased 11-fold between 1990 and 2010, and nearly 90 percent of people who try heroin for the first time these days are white," the Atlantic noted. Washington County in Pennsylvania, for instance, logged eight overdoses in 70 minutes earlier this year. There have been more than 50 fatal overdoses this year.

2. Alcohol poisoning. More than 2,200 Americans die a year after consuming too much alcohol and three-fourths are ages 35 to 64, according to the Centers for Disease Control and Prevention. According to the Daily Mail, "While death rates related to drugs, alcohol and suicides have risen for middle-aged whites across the board, the largest surge are seen among those with the least education. For those with a high school degree or less, deaths caused by drug and alcohol poisoning rose four fold, suicides increased by 81 per cent, and deaths caused by liver disease and cirrhosis jumped 50 per cent."

3. Suicide. The Case-Deaton study noted that poisonings overtook lung cancer as a leading cause of death in 2011 in this age group and that suicide rates were headed in that direction, as well.

4. The end of the American dream? With the disappearance of stable jobs in manufacturing and construction over the years, many people with only a high school education may not have as many opportunities as they may have had a generation ago. Surveys have shown that about half of middle-aged Americans have not been able to save enough money for retirement. Many in this generation are "the first to find, in midlife, that they will not be better off than their parents," the authors wrote.

5. Breakdown of family support networks. The divorce rate among those over age 50 has doubled — to one out of every four people — at a time when divorce rates for other age groups have stabilized or dropped. In a new book about white America, Charles Murray shows how "marriage has become the fault line dividing America's classes." As compared with the white working class, the white affluent divorce less, report that they have happier marriages and raise fewer children as single parents, according to the New Republic.

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