Is the proposed explanatory factor changing in the relevant way?
Suppose, for instance, that people who often move have shallower community roots.
That could be an important part of the answer to our mystery only if residential mobility
itself had risen during this period.
Is the proposed explanatory factor vulnerable to the claim that it might be the result of civic disengagement, not the cause?
For example, even if newspaper readership were closely correlated with
civic engagement across individuals and across time, we would need to weigh the possibility that reduced newspaper circulation is the result (not the cause) of disengagement. Against that set of benchmarks, let us consider various potential influences on social capital formation.