although some scholars of the jim crow period agree that the 20th-century notion of invisible blackness shifted the color line in the direction of paleness, and "expanded" the labor force in response to southern blacks' great migration to the north. but, others (such as the historians joel williamson, c. vann woodward, george m. fredrickson, and stetson kennedy) considered the one-drop rule a consequence of the need to define whiteness as being pure, and justifying white-on-black oppression.