Why has the Natural Rate of Unemployment Increased over Time?

In 1970, when Robert Hall asked, "Why Is the Unemployment Rate So High at Full Employment?" the unemployment rate for adult men stood at 3.5 percent. That rate, which had been substantially below that level throughout the late 1960s, would climb to 4.4 percent in the recession of 1971. More recently, after the longest economic expansion of the post-war period, the unemployment rate of prime-aged men in the late 1980s settled at just below 5 percent of the labor force. What changes in the American labor market led to this apparent secular increase in the natural rate of unemployment. Twenty years later, we revisit Hall's question and turn up some new answers.