Yet there is deep uncertainty about how the pattern will play out now, as two trends are interacting. Artificial intelligence has become vastly more sophisticated in a short time, with machines now able to learn, not just follow programmed instructions, and to respond to human language and movement.

At the same time, the American work force has gained skills at a slower rate than in the past — and at a slower rate than in many other countries. Americans between the ages of 55 and 64 are among the most skilled in the world, according to a recent report from the Organization for Economic Cooperation and Development. Younger Americans are closer to average among the residents of rich countries, and below average by some measures.

Clearly, many workers feel threatened by technology. In a recent New York Times/CBS News/Kaiser Family Foundation poll of Americans between the ages of 25 and 54 who were not working, 37 percent of those who said they wanted a job said technology was a reason they did not have one. Even more — 46 percent — cited “lack of education or skills necessary for the jobs available.”

Self-driving vehicles are an example of the crosscurrents. They could put truck and taxi drivers out of work — or they could enable drivers to be more productive during the time they used to spend driving, which could earn them more money. But for the happier outcome to happen, the drivers would need the skills to do new types of jobs.

The challenge is evident for white-collar jobs, too. Ad sales agents and pilots are two jobs that the Bureau of Labor Statistics projects will decline in number over the next decade. Flying a plane is largely automated today and will become more so. And at Google, the biggest seller of online ads, software does much of the selling and placing of search ads, meaning there is much less need for salespeople.