One-drop rule

The one-drop rule is a social and legal principle of racial classification that was historically prominent in the United States asserting that any person with even one ancestor of sub-Saharan-African ancestry ("one drop" of black blood) is considered black (Negro in historical terms). This concept evolved over the course of the 19th century and became codified into law in the 20th century. It was associated with the principle of "invisible blackness" and is an example of hypodescent, the automatic assignment of children of a mixed union between different socioeconomic or ethnic groups to the group with the lower status.

The same principle may be applied, or has been applied in opposite ways in different cultures, as is the case in Brazil vs. the United States.

Before and during the centuries of slavery, people had interracial relationships, both forced and voluntarily formed. In the antebellum years, free people of mixed race (free people of color) were considered legally white even if individuals had up to one-eighth or one-quarter African ancestry (depending on the state). Many mixed-race people were absorbed into the majority culture based simply on appearance, associations and carrying out community responsibilities. These and community acceptance were the more important factors if a person's racial status were questioned, not his or her documented ancestry. Because of the social mobility of antebellum society in frontier areas, many people did not have documentation about their ancestors.

Based on DNA and historical evidence, Thomas Jefferson is widely believed to have fathered the six mixed-race children of his slave Sally Hemings; four survived to adulthood. Hemings was three-quarters white by ancestry and a half-sister of Martha Wayles Jefferson. Their children were born into slavery because of her status; as they were seven-eighths European in ancestry, they were legally white under Virginia law of the time. Jefferson allowed the two oldest to escape in 1822 (freeing them legally was a public action he elected to avoid); the two youngest he freed in his 1826 will. Three of the four entered white society as adults, and all their descendants identified as white.

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