It is a real metric. If you track most "white" americans back, most come from immigrants in the last 1-2 generations. Most black americans can trace lineage back to africans who came to the continent in the 1600-1700s, then mixed with Euro and Native people, and created black americans, who have deep roots in this country. I Traced mine back to a man born in 1650 Massachusetts. The dudes in my family did marry a lot of immigrant chicks over the years though - but the main lineage is solid back to 1650.