Recent public health interventions may also be giving poor children a health boost. We’ve known for decades that smoking is bad for people’s health, but richer Americans were quicker to quit than their poorer neighbors. That means that poorer, older Americans are more likely to have smoked at some time in their life, and probably had more exposure to secondhand smoke from their parents when they were young. As smoking rates have declined among lower-income groups, and fewer parents smoke around their children, the poor may see benefits from having less lifetime exposure to smoke.

Other forms of air pollution that tend to disproportionately affect poor children have also declined. So has the frequency of fatal accidents, another major cause of death for children. The death of infants in their first year of life, while higher in the United States than in many other countries, has fallen thanks to a number of changes in medical practices.

The largest mortality declines were for black boys. The paper documenting the reductions, published by the National Bureau of Economic Research, called the change in death rates for African-American boys and men “stunning declines.” Research on more recent mortality trends shows continued reductions in death rates among black Americans since 2010.

Most of the recent research on life expectancy is calculated using information about Americans who have already reached middle age. In many ways, that’s a sensible approach; most deaths occur among the old.

A recent landmark study of middle-aged people, covered extensively in The New York Times and elsewhere, used a trove of income records to link Americans’ age of death with their hometown and precise earnings. That study found a growing divide in life expectancy between rich and poor Americans, though there were some places that were better than others. In order to use that rich data, the researchers needed to look at the longevity of Americans who had already reached the age of 40. That means that the trends did not capture changes in health patterns that might be affecting younger Americans.

The new research on children was conducted using a less precise method. Instead of tracking each child, Ms. Currie and her co-author, Hannes Schwandt, an assistant professor of economics at the University of Zurich, looked at the counties in the United States with the greatest concentrations of poor and rich children. By examining the rates that children died in those places over time, they were able to infer differences in the death rates of children at different places on the economic ladder.