As the workforce ages in some countries and middle-aged workers are in short supply, industrial robots are stepping in to perform their jobs, according to a recent paper by economists Daron Acemoglu and Pascual Restrepo.

The big picture: Today, Asian countries have about eight workers for every person over the age of 65, and Europe and North America have 4 or fewer, according to the UN. But by 2050, seven Asian countries, 24 European and four Latin American will have a ratio of less than 2 workers per retiree, the UN projects. Experts say the U.S. social security system is stable at a ratio of 3 workers per retiree. When it falls to 2 to 1, they say the program becomes unsustainable.

That brings in robots: Germany, Japan and South Korea — all of which are aging rapidly — are seeing a comparatively fast rise in the use of industrial robots and automation. The U.S. and U.K. are "lagging behind in robotics because they are not aging as rapidly" Acemoglu and Restrepo write in a National Bureau of Economic Research working paper.

Japan had 14.20 industrial robots per 1,000 manufacturing workers in 2014, versus 9.14 in the U.S.

per 1,000 manufacturing workers in 2014, versus 9.14 in the U.S. Worldwide, there were an average of 74 industrial robots installed per 10,000 manufacturing employees in 2016, a jump from 66 the year before, according to a February report from the International Federation of Robotics.

Aging, Acemoglu and Restrepo report, can account for 40%-60% of the difference between nations' adoption of industrial robots. "[I]f the United States had the same demographic trends as Germany, the gap in robotics between the two countries would be 25% smaller," they wrote.

Key assumption: Middle-aged workers more so than older ones "specialize in production tasks that can be automated using industrial robots." These tasks include welding, painting, assembly, machining, and loading.

The rapid progress and declining cost of technology also play a role in the adoption of automation, says Guy Michaels, an economics professor at the London School of Economics.

What's next: "People will adjust," says Michaels, who points to the generational aspect of technology being adopted. "Young people are probably readier to accept technology."