Part of this is a fluke of the business cycle. Because of the Great Recession, which walloped the construction industry, jobs in various building trades will have to surge just to get back to normal levels of employment. But there's more involved in the process than that.
The main reason the low and high ends of the job market are growing is that the middle is getting squeezed. Different explanations are put forward for this: government policy, the decline of unions, offshoring jobs. But the prevailing theory is that middle-skill positions are disappearing because of technology.
The process has been going on for 30 years, but it has been largely obscured because it happens in fits and starts. The theory, as propounded by David Autor, an economist at the Massachusetts Institute of Technology (MIT) in Cambridge, is that the rapid spread of low-cost computing power has replaced many "routine" jobs. These workers accomplish a set of specific activities by following defined procedures, which a computerized device can mimic. Think automated teller machines instead of bank tellers, robots instead of factory workers, computerized answering systems instead of customer service representatives.
But computers aren't good at handling nonroutine jobs, whether it's managing a work group or cleaning up a mess in an elementary school. So they don't eliminate jobs at the top end of the skill spectrum, such as research scientists and managers. They also don't replace low-skill nonroutine occupations, like janitors.
"This process is going to gain more and more steam," says Nir Jaimovich, an economist at Duke University in Durham, N.C. His research shows that almost all the loss of these routine jobs happens during recessions – and they never come back during the recovery. So the Great Recession has hit the reset button on these occupations. In 1981, routine positions accounted for 58 percent of all jobs; by 2011, they were down to 44 percent.