The adoption of this systems approach leads us to develop and test theory about the effects of flows
over time, because they are related to the complex coevolution of variables as the system responds
and reacts to change. These temporal effects are distinct from commonly modeled, temporal factors,
such as learning curves, in which the rate of change in a single variable is non-linear over time. Our
theory about human capital flows includes the actions and reactions of each component in the system
because we examine and compare system responses to departures and additions, and develop theory about how the system responds to changes.