Here's a somber thought: The United States may never — or at least not anytime soon — regain "full employment," meaning an unemployment rate between, say, 4 percent and 5.5 percent. It is now four years from the recovery's start, and the number of jobs is still 2.2 million below the pre-recession peak. Since World War II, this has never happened. After the harsh 1981-82 recession, employment recouped lost ground in 12 months.

Economists are searching for an explanation, and one recent candidate seems surprising: high tech.

It's usually seen as an engine of growth, but the spread of automated processes and robots has actually acted as a drag on job creation and has kept the unemployment rate high (7.6 percent in June), argue Erik Brynjolfsson and Andrew McAfee of the Massachusetts Institute of Technology. Digital technologies, they contend, have enabled companies to cut costs, increase productivity (such as efficiency), improve profits and slash payrolls. They expect more of the same.