Two reasons why natural rate of unemployment changed in the US. for the past 50years?

In my opinion, it hasn%26#039;t changed much. The unemployment rate in the late 1950%26#039;s, a time of economic expansion, was in the low 4% range. Today, as the U.S. enters a recession, it%26#039;s 4.8%.

You can see the unemployment rate over the last fifty years at:

research.stlouisfed.org/fred2/series/U... - 15k -