What Happened?

[BrianTH]

You count as part of the labor force if you are either employed or actively seeking employment, whether or not you are also getting some form of social assistance. People drop out of the labor force when they are no longer actively seeking employment--those are the "hidden" unemployed in a typical recession, because if conditions improve they might start looking again.

The evidence that the increase in the Pittsburgh labor force was the result of in-migration is basically that it started increasingly immediately (before, in fact, employment started to dip in Pittsburgh), such that the labor force is bigger now than it was before the recession. In some other places, the labor force decreased, and then has started increasing again, which is the process I noted above (people being discouraged from looking, then starting to look again). But it is hard to explain the labor force actually growing in size from before the recession without a population increase, and it was too rapid to be the result of births.