In 1973, 24% of all U.S. wage and salary workers were members of unions. Since then, organized labor participation has steadily declined. Over the past decade, nationwide union membership declined 7.1%.

Despite the decline, some states remain union strongholds. Nearly one-quarter of New York’s employed population was a member of a union, the highest proportion in the nation. In North Carolina, on the other hand, less than 2% of workers participated in labor organizations, the lowest share in the country.

Click here to see the states with the strongest unions

Click here to see the states with the weakest unions

The long-term decline in the unionization of American workers is due to a range of factors, explained David Macpherson, E.M. Stevens Professor of Economics and Department Chair at Trinity University. For one, heavily unionized industries such as manufacturing and construction have shrunk over the years due largely to globalization, deregulation, and more competition. With U.S. companies having to compete on the global market, “the high union wages have actually helped contribute to that shrinking because they’re less competitive with [wages] in the rest of the world,” said Macpherson.

Other forces have also been at work. Mcpherson added that the average company size has decreased somewhat across the country, which makes it harder for workers to organize. More and more companies are also moving to the south and west, which generally have laws less favorable for unions. In addition, “there’s been decreased confidence in the ability of unions to increase the welfare for workers,” according to Mcpherson.

The recent economic downturn may partly explain the weakening of American unionization. Mcpherson said union members are more likely to get laid off during economic downturns because their wages are often based on fixed multi-year contracts. Ironically, with less wage flexibility, employers often lay off unionized workers instead of lowering their wages.

While the relationship between union participation and the labor market is complex, the 2014 unemployment rates in eight of the 10 states with the strongest union participation rates exceeded the national unemployment rate of 6.2%. The unemployment rates in seven of the 10 states with the weakest union participation, on the other hand, were below the national rate.

Of course, there are also tangible benefits for the union members themselves and all the workers covered by collective bargaining agreements. There are the obvious benefits, which include the ability to bargain for a higher wage and better working conditions.

There are also benefits for employers, while many believe union work rules can hurt productivity, Mcpherson explained that worker turnover is far lower among union members, which lowers hiring costs for companies. More experienced workers further lower costs by training younger workers on the job.

Labor organization is an extremely controversial subject, and the prevalence of unions is often a reflection of how people and politicians feel about the issue. Many of these states are expressly anti-union. Today, 24 states prohibit union security agreements, restrictions known as right to work laws. All of the states with the lowest union membership had right to work laws, while none of the states with the highest union membership has such laws. As Mcpherson noted, however, right to work laws and union membership are likely not causally related. The failure of unions to flourish is less due to the right-to-work laws themselves, and more to do with the climate that gave rise to those laws in the first place.

Based on figures published by Unionstats.com, an online union membership and coverage database, 24/7 Wall St. identified the states with the highest and lowest union membership as a percentage of total employment. The database, which analyzes Bureau of Labor Statistics’ (BLS) Current Population Survey, provides labor force and union membership figures in both the public and private sector, including manufacturing and construction. Additionally, 24/7 Wall St. reviewed annual average unemployment rates for each state from the BLS, as well as income and poverty data from the 2013 American Community Survey, produced by the U.S. Census Bureau. Rates of change were calculated over 10-year periods, from 2005 through 2014.

These are the states with the strongest and weakest unions