It is often suggested that the key reason for this is that the physical differences between these other groups and whites are less pronounced than the physical differences between African blacks and whites, and therefore are less threatening to whites . However, keep in mind that the one-drop rule and anxiety about passing originated during slavery and later received powerful reinforcement under the Jim Crow system. For the physically visible groups other than blacks, miscegenation promotes assimilation, despite barriers of prejudice and discrimination during two or more generations of racial mixing. As noted above, when ancestry in one of these racial minority groups does not exceed one-fourth, a person is not defined solely as a member of that group. Masses of white European immigrants have climbed the class ladder not only through education but also with the help of close personal relationships in the dominant community, intermarriage, and ultimately full cultural and social assimilation.
Young people tend to marry people they meet in the same informal social circles.
For visibly non-caucasoid minorities other than blacks in the United States, this entire route to full assimilation is slow but possible. For all persons of any known black lineage, however, assimilation is blocked and is not promoted by miscegenation. Barriers to full opportunity and participation for blacks are still formidable, and a fractionally black person cannot escape these obstacles without passing as white and cutting off all ties to the black family and community. The pain of this separation, and condemnation by the black family and community, are major reasons why many or most of those who could pass as white choose not to. Loss of security within the minority community, and fear and distrust of the white world are also factors.