Margo R A
Econ Lett. 1989 Dec;31(4):403-6. doi: 10.1016/0165-1765(89)90038-4.
"Evidence from the 1940 [U.S.] census reveals that southern-born blacks who left the region prior to 1935 had higher incomes than non-southern born blacks. The income gap between migrants and non-migrants declined with schooling."