Over the last 15 years, unionization rates of workers fell below 10 percent in nine of the Rocky Mountain states.

The only regional holdout is Montana, with a unionization rate of 15 percent.

Wages and unionization rates for manufacturing jobs also decline.

With the nation's lowest unionization rate, North Carolina is probably the least likely place to expect union solidarity.

Dropping unionization rates cannot be attributed entirely to changing market structures.

Korea's unionization rate peaked in 1989 at 19.8% and fell to 10% 2004.

In the public sector, where there is little competition, unionization rates remain at more than 35 percent.

Despite small jumps and declines from year to year, the overall unionization rate has fallen compared with recent decades.

Last year's 13.5 percent unionization rate was down from 20 percent in 1983 and a peak of 35 percent in the 1950's.

"What happened last year is a continuation of a long-term decline in the unionization rate," Professor Troy said.