The natural unemployment rate in the United States has varied over the last 50 years. According to the Congressional Budget Office, the natural rate was 5.5% in 1960, rose to about 6.5% in the 1970s, and had declined to about 4.8% by 2000. What do you think might have caused this variation? ... Are there any alternative perspectives to consider? Thank you.

Question from Concept Problems Chapter 16 pg 681, https://my.uopeople.edu/pluginfile.php/1274764/mod_resource/content/1/TEXT%20macroeconomics-principles-v2.0.pdf