there is certain good in diversity which comes from immigrants from different locations altogether with skills and natural resources it was a factor that allowed US to build the most powerful economy in the world. However, times change. I remember from lessons of History in the Univesity that every civilization has phases: Establishing, Prosperity, Decay. It is obvious that average impact at each of the three phases might be different.
Also it might be that in the North and in the South the impact is opposite.