The expansion of the last nine years has been steady and successful at putting millions of people to work, but it has fallen short in overall growth or in generating substantial boosts to incomes.

The culprit: productivity. The amount of output per hour of labor has been growing slowly. In the last five years, it has risen about 1 percent annually, compared with 2.1 percent on average since 1947.

If that changes, and businesses find ways to get more production of goods and services out of each hour of work, it should set the stage for higher wages and faster growth. That’s what happened in the 1990s: Annual productivity rose an average of 1.8 percent from 1991 to 1995, then leapt to a 3 percent annual rate from 1996 to 2000.

What are the chances of a similar jump in the coming years, with similar beneficial results? Economists don’t have a great understanding of what drives productivity advances, but there are three realistic possibilities of where gains might come from — and they aren’t mutually exclusive.

Solow’s Paradox and Technological Diffusion

In 1987, the economist Robert Solow joked that “you can see the computer age everywhere but in the productivity statistics.” That is, even as there were remarkable advances in semiconductors, software and desktop computing, it wasn’t evident in any efficiency improvements in the overall economy.

The surge didn’t arrive until well into the 1990s, and that pattern — a long lag between a technological innovation and its full economic impact — makes sense when you think about it.

For example, fewer corporate executives have a full-time secretary now than they did in the 1980s, in part because of innovations like voice mail, email and desktop word processors. But it’s not as if companies suddenly fired a bunch of secretaries on the day they got a voice-mail system. It was a gradual process as more people became accustomed to answering their own phones and typing their own memos.