For hundreds of years, people have been talking about machines taking jobs from people. Less often discussed: machines creating new jobs.

In the first part of the 20th century, agricultural technology — the tractor, chemical fertilizers — meant a single farmer could suddenly grow much more food. So we didn't need as many farmers. Technology destroyed a huge number of farming jobs.

But that technology also made food cheaper, so people had more money to spend on other things, like TVs and radios and newly invented appliances. Factory jobs boomed. Other sectors grew as well; the midcentury economy had lots of midskill white-collar jobs like secretary and bookkeeper.

The next wave of major technological change came in the latter part of the 20th century. Robots in factories and computers in offices automated away many of the jobs that earlier technologies had created in the earlier part of the century. You can see the rise and fall of these sectors in the graph below.

In the past several decades, as factories became more automated, manufactured goods got cheaper. That left many Americans with more money to spend on services — eating out more, spending more on health care and education. These sectors are still labor-intensive and have continued to add jobs. But that may change as computers get smarter and more ubiquitous.

Finally, there are a few — frankly, totally unsurprising — sectors where jobs have been disappearing for well over a century now. At no point in the 20th century was it a good time to be a blacksmith.

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