One of the most worrying economic trends over the past few decades has been the decline of middle-class jobs in the U.S.

As “routine” jobs—often middle-class work based on a relatively narrow set of repeated tasks, such as welding-machine operators or bank tellers—disappear, many workers who would typically have held them have taken on lower-paying low-skill manual work or simply dropped out of the labor force, according to new research from a trio of economists.

The paper, called “Disappearing Routine Jobs,” provides more evidence that the transformation of work in the U.S.—from an industrial economy to a digital one where routine work is automated or outsourced and the remaining jobs are concentrated in low-paid service work or high-skilled knowledge work—is contributing to the shrinking labor-force participation rate, said co-author Henry Siu, a professor at the University of British Columbia.

The share of Americans working in routine jobs has fallen from 40.5% in 1979 to 31.2% in 2014, according to the paper. The federal government’s official measure of Americans age 16 and over who are working or seeking work has fallen from a recent high of 67.3% in 2000 to 62.7% in November 2016.

“Routine jobs are disappearing and more and more prime-age Americans aren’t working,” said Mr. Siu. “These things are two sides of the same coin.”