The June US jobs report is out, and while the unemployment rate is down, part of the fall in unemployment came from a 0.3 percentage point drop in the labour force participation rate to 62.6%, the lowest rate since October 1977. Americans are considered to be in the labour force by the Bureau of Labour Statistics if they are employed or actively looking for work.
The labour force participation rate steadily grew between the 1970s and the 1990s, reaching its peak of 67.3% in 2000. During the 2000s, and especially since the Great Recession, the participation rate began to drop. Part of that drop was in response to the economic crisis that started in 2008, and part of the drop comes from demographic factors like the ageing of the US population and the retirement of the baby boomers.