Racial categories are inevitably problematic, because they are ostensibly biological—that is, they are supposed to be based on heritable physical characteristics like skin color—but in fact are heavily cultural, as demonstrated by the infamous “one drop” rule in the nineteenth-century southern United States, which proclaimed that men and women were Negroes, even when they could not be distinguished by whites from appearance, if any of their ancestors, no matter how remote, were African.