The rise and decline of private sector unionization were among the more important features of the U.S. labor market during the twentieth century. Following a dramatic spurt in unionization after passage of the depression-era National Labor Relations Act (NLRA) of 1935, union density peaked in the mid-1950s, and then began a continuous decline. At the end of the century, the percentage of private wage and salary workers who were union members was less than 10 percent, not greatly different from union density prior to the NLRA.
Hirsch, B.T. & Schumacher, E.J. (2001). Private sector union density and the wage premium: Past, present, and future. Journal of Labor Research, 22(3), 487-518. doi: 10.1007/s12122-001-1018-7
Journal of Labor Research