U.S. union membership over time
The above chart is extremely useful in understanding labor-relations trends over the past century or so in the United States. As you can see, the 1930s, during the Great Depression, is when unions began to grow in prominence and power. This held strong for many years, and the decline of unions is a very recent trend in labor relations. Many correlate this decline in unions with the economic struggles of 2008–2009, noting that the rise and fall is between the two worst economic collapses in history.
Source
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"Union Membership in the U.S. 1930-2010."
http://upload.wikimedia.org/wikipedia/commons/2/25/Union_membership_in_us_1930-2010.png
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