WASHINGTON - Pay for globe-trotting CEOs has soared, even as most workers remain grounded by paychecks that are barely budging.

While pay for the typical CEO of a company in the Standard & Poor's 500 index surged 9 percent last year to $10.46 million, it rose a scant 1.3 percent for U.S. workers as a whole. That CEO now earns 257 times theU.S. average, up from a multiple of 180 in 2009, according to an analysis by the Associated Press and Equilar.

Those figures help reveal a widening gap between the ultra-wealthy and ordinary workers. That gap has fed concerns about economic security - everywhere from large cities where rents are high to small towns where jobs are scarce.

Here are five reasons why many workers are stuck with stagnant incomes.

Robots: Millions of factory workers have lost their spots on assembly lines to machines. Offices need fewer secretaries and bookkeepers in the digital era.

Robots and computers are displacing jobs that involve routine tasks, according to research by David Autor, an economist at Massachusetts Institute of Technology. As these middle-income positions vanish, workers are struggling to find new occupations that pay as much.

High unemployment: The aftermath of the recession left a glut of available workers. Businesses face less pressure to give meaningful raises when a ready supply of job seekers is available. They're less fearful that their best employees will defect to another employer.

The current 6.3 percent unemployment rate isn't so low that employers will spend more to hire and keep workers. Wages grew in the late 1990s when unemployment dipped to 4 percent, a level that made high-quality workers scarce and compelled businesses to raise pay.

Globalization: Companies can cap wages by offshoring jobs to poorer countries, where workers on average earn less than the poorest Americans. Consider China. A typical Chinese factory employee made $1.74 an hour in 2009, according to the Bureau of Labor Statistics - roughly a 10th of what their U.S. counterparts made.

Some analysts say this trend may have peaked. But many economists say the need for the U.S. to compete with a vast supply of cheap labor worldwide continues to exert a depressive effect on U.S. workers' pay.

Weaker unions: Organized labor no longer commands the heft it once did. More than 20 percent of U.S. workers were unionized in 1983, compared with 11.3 percent last year, according to the Bureau of Labor Statistics. That's drastically reduced union influence. Result: Fewer workers can collectively negotiate for raises.

Low inflation: For the past five years, the consumer price index has averaged an ultra-low 1.6 percent. When inflation is high, employees tend to factor it into pay raises. When inflation is as low, it almost disappears as a factor in pay negotiations.