Where do jobs come from? It's a simple question with vast implications. This article previews findings from a forthcoming report by the McKinsey Global Institute.

For the second straight summer, the U.S. recovery hit a wall. Unemployment is on the wrong side of 9 percent. The velocity of job creation is one-third of the pace we need to keep up with our growing population. To understand why, let's start at square one: Where do jobs come from?

That's no childish question, but it deserves a simple answer. A job is born when demand grows faster than productivity, either because consumers buy more of the same stuff or because innovations create new stuff.

Even simpler: Jobs are the difference between real economic growth and productivity. When the economy grows, jobs tend to grow in tandem. But when productivity grows faster, companies can make do with fewer people. You can sum this up with an easy equation:

Employment growth = economic growth (-) productivity growth.

Apply that equation to two industries: manufacturing and health care. In the last 30 years, manufacturing jobs have declined (see the graph below) but output has grown as steadily as almost any sector in the country. What's making up the difference? Productivity. Labor productivity in manufacturing grew at a whopping 5 percent annualized between 1980 and 2010. That's almost twice as fast as any other industry in the country.

In health care, the opposite has been true. Employment has grown faster in health care than any other large sector, even in the recession. This is because demand has increased rapidly due to an aging population with wider treatment options, while productivity gains have been limited or even negative. McKinsey rounds up the largest sectors in the handy chart below.

Productivity is a good thing for America, but it's not always great for American workers. Since 2000, employment grew fastest where productivity grew slowest (health care) and jobs grew slowest where productivity grew fastest (manufacturing).