Today, we learned that the unemployment rate rose to 9.1%, even though the net number of jobs in the economy increased. That might seem odd -- how can unemployment and employment both rise at the same time? In some months, this has been attributable to data discrepancies, but in May this occurred because 105,000 more Americans who were sitting on the sidelines entered the workforce. This has actually hasn't happened very often while the unemployment rate has been high. Let's look at a little history.
Below you will find a chart showing the number of people exiting the workforce or entering from the sidelines (as opposed to through population growth). For example, in May, the population rose by 167,000 people, but the workforce increased by 272,000 people. In other words, the more people entered the workforce than can be accounted for by the increase in population. That number, 105,000, reflects Americans who had exited the workforce but reentered in May.
You'll see that this number is generally negative. In those months, this means that the population growth exceeded the growth of the workforce. In some of those months, the workforce size actually contracted. In either situation, this indicates that additional people are exiting the workforce. The pool of Americans sitting out of the job market grows larger when this occurs.