Binary Anxiety

Noble's essay spoke to my interest in racial passing, the failings of a reliance on racial visibility/physiognomy to verify racial "fact," and the systematic (or "scientific") creation of American black identity by whites in positions of power. 

The one-drop rule and segregation were used to secure the borders of difference due to white anxieties over racial mixing and the disappearance of social hierarchies of meaning. If meaning is dependent upon difference (e.g. 'black' only has significance in relation to its opposite, 'white'), and blackness (as a "pure" racial category) was to slowly become extinct through racial mixing, whiteness would lose all its value. Thus, 'mulattoes' were/are a threat to social meaning and cultural order in their inability to conform to binary racial meaning. Passing was especially threatening given that it could trespass these boundaries and attest to the unreliability of racial visibility. In an attempt to regulate meaning and preserve white purity rigid binary structures (Jim Crow laws, Black Codes/Anti-miscegenation laws) needed to be erected to ameliorate white fears of the erasure of racial meaning/white superiority. Segregation ensured that whiteness would remain a distinct category while opening up "colored" to all non-whites and ensuring that all mixed-folk identify with their "dominant" side, or their nonwhite ancestry. There could be no such thing as 'mixed whites.' As Nobles puts it, "blacks and other nonwhites were mixed; whites were not" (54). 

How did white anxieties around the categorization/ pseudo-scientific understanding of blackness and obsessions with racial visibility facilitate the use of  census-science, segregationist public policy, and the historical endurance of the one-drop rule?  

Why did American scientists and legislators feel so deeply committed to preserving racial binary distinction? What use did obsessively delineating the terms of blackness hold for constructing whiteness as a pure category and maintaining white dominance?