A sharp rise in death rates among white middle-aged Americans has stunned the researchers who conducted a new Princeton study released Monday.

The formerly overlooked trend has hit less-educated 45- to 54-year-old Americans the hardest, the Guardian reported, adding that the increased deaths are largely thought to be a result of more suicides, the misuse of drugs and alcohol, and greater financial stresses.

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“This was absolutely a surprise to us. It knocked us off our chairs,” said Anne Case, an economics professor at Princeton University who worked on the study, in the Guardian. “We wanted to make sure we weren’t missing something. Everyone’s been stunned.”

The study shows that the death rate for whites aged 45 to 54 in the U.S. with a high school degree or less rose by 134 deaths per 100,000 people from 1999 to 2014, the New York Times reported, adding that, in total, middle-aged whites have a mortality rate of 415 per 100,000.

The turnaround reverses decades of falling mortality rates achieved through better medical care and lifestyle choices that continue to improve in other groups in the U.S. and in other nations around the world, the Guardian reported.

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Increases in mortality rates in middle-aged whites rose in parallel with their increasing reports of pain, poor health and distress, economist Dr. Angus Deaton explained in the Times article, saying, "Only HIV /AIDS in contemporary times has done anything like this.”

The Guardian agreed with his statement, claiming that the death toll related to the increased mortality rate is comparable to the 650,000 Americans who lost their lives during the AIDS epidemic starting in 1981.

Samuel Preston, a professor of sociology at the University of Pennsylvania, was quoted by the Times: “This is a vivid indication that something is awry in these American households.”