In ordinary reality, it seems obvious that most people belong to one or another of a few major races due to biological differences. Actually, human biological sciences have failed to identify any physical essences that distinguish a race; and there are no stable physical traits that all members of any race share. For example, some black people have lighter skin hues than some white people, and overall there is greater variation of so-called racial traits within races than between races. When the human genome was mapped at the turn of the twenty-first century, geneticists reporting on the research emphasized that they had found no genes for race.

Of course, the physical traits that count as racial are genetically inherited, but there is no difference in principle between those traits and others. Both globally and historically, criteria for racial membership have varied. In colonial times, a person was considered white if most of their great grandparents were white. By 1900, the “one-drop” rule was in effect throughout the land: a person was considered black if there were any black ancestors, no matter how far back they were. The one-drop rule erased positive racial identities for Americans with both black and white ancestry—they were, and to a large degree still are, considered black, rather than multiracial, mixed race, or biracial.

The lack of a biological foundation for black or white racial identity has led some writers to suggest either that racial categories be eliminated or that racial identities be recognized as purely social. On that social basis, there is no rational reason why people with both black and white ancestry should not be recognized as mixed race, instead of automatically assigned to the black category. Others have tried to reconstruct less rigorous “biological” bases for race, and still others have argued that, within the African American tradition, race has always been understood to involve something more than biology.