If you believe in the law of supply and demand — and I do — there’s not much mystery to the stagnation of wages even though unemployment is falling. In the past year, the economy has added 2.6 million jobs; the unemployment rate has declined to 5.9 percent. But despite the extra jobs, too many potential workers must be chasing too few openings. So wage gains are meager, and the recovery suffers. Case closed? Not really. We know what’s happening, but not why. That’s the intriguing question.

One theory is that there are lots of potential workers ready to pile back into the labor market. It was so weak for so long that there are 6 million people, says the Bureau of Labor Statistics, who say they want jobs but stopped looking. To these must be added the 9.3 million officially unemployed and another 7 million part-time workers who say they’d like full-time work. All these potential workers keep the balance of supply and demand in employers’ favor, the argument goes.

This seems plausible, though as yet evidence of hordes of workers returning to the job hunt is skimpy. But even if this theory is vindicated, I suspect something else is also involved: Workers are so insecure that they’re afraid to abandon their present jobs for something better; therefore, companies don’t have to pay higher wages to retain them. Not surprisingly, labor compensation — wages and fringe benefits — has barely kept pace with inflation at about 2 percent annually since mid-2009.

Bargaining power has shifted to employers, based on workers’ fears of permanent job loss. Now comes a new study from Macroeconomic Advisers, a major forecasting firm, suggesting just that. Since 2000, it notes, a shrinking share of workers has changed jobs. In economics jargon, labor markets have “less fluidity”; workers are “moving less among jobs” than before. By reducing turnover, this defensiveness relaxes pressure on employers to increase compensation.

The study finds the tendency of people to shift jobs has dropped by almost one-third, from about 13 percent of the 16-and-over population in 2000 to 9 percent in 2013. Slower growth in compensation has, in turn, reduced labor’s share of the national income. Since World War II, the labor share has generally fluctuated between 62 percent and 66 percent of gross domestic product (GDP), the study says. In 2014, it’s 58 percent. (The difference between a 58 percent and a 62 percent share is about $700 billion in lost annual income. National income also includes corporate profits, small-business and farm income, interest, rents and some government.)

Traditional explanations of labor’s smaller slice of national income have focused on weaker labor unions and a low minimum wage. But the deeper causes may lie in widely felt anxieties about job security, based on new technologies, globalization (especially the ability of companies to move production offshore), the fear of losing health insurance and the danger of another financial crisis. An aging population may magnify these worries.

Older people “become more ingrained in their jobs,” says economist Ben Herzon of Macroeconomic Advisers, the study’s author. “They get settled [in their communities], and they don’t want to move.” Another factor strengthening businesses’ bargaining power is the gradual increase in the proportion of employment represented by large firms, he argues. In 1992, firms with more than 500 employees represented 42 percent of all workers; by 2013, their share was 46 percent.

There’s a historical twist here. In the 1970s, companies were said to hoard workers during economic downturns — they kept workers they didn’t immediately need, because they assumed that the economy would soon revive and they’d be better off with experienced employees rather than having to hire and train new ones. Now, the mindset seems just the opposite: Who knows what the future holds? So companies seem quick to fire and slow to hire. That fear may be tying workers to existing jobs.

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