Something startling is happening to middle-aged white Americans. Unlike every other age group, unlike every other racial and ethnic group, unlike their counterparts in other rich countries, death rates in this group have been rising, not falling.

That finding was reported Monday by two Princeton economists, Angus Deaton, who last month won the 2015 Nobel Memorial Prize in Economic Science, and Anne Case. Analyzing health and mortality data from the Centers for Disease Control and Prevention and from other sources, they concluded that rising annual death rates among this group are being driven not by the big killers like heart disease and diabetes but by an epidemic of suicides and afflictions stemming from substance abuse: alcoholic liver disease and overdoses of heroin and prescription opioids.

The analysis by Dr. Deaton and Dr. Case may offer the most rigorous evidence to date of both the causes and implications of a development that has been puzzling demographers in recent years: the declining health and fortunes of poorly educated American whites. In middle age, they are dying at such a high rate that they are increasing the death rate for the entire group of middle-aged white Americans, Dr. Deaton and Dr. Case found.

The mortality rate for whites 45 to 54 years old with no more than a high school education increased by 134 deaths per 100,000 people from 1999 to 2014.