In societies that regard some races of people as dominant or superior and others as subordinate or inferior, hypodescent is the automatic assignment of children of a mixed union or mating between members of different socioeconomic groups or ethnic groups to the subordinate group. The opposite practice is hyperdescent, in which children are assigned to the race that is considered dominant or superior.

Parallel practices include agnatic descent, matrilineality and cognatic descent, which assign race according to the father, mother, or some combination, without regard to the race of the other parent. Since either parent (or both) might be of mixed-race, hypo- and hyper-descent can operate in tandem with, or separately from, a system of agnatic or cognatic racial assignment.

The American practice of applying a rule of hypodescent began its development in the colonies, as slavery was established. But, it developed in its most strict legal definitions and application after the end of slavery in the early 20th century. After the Reconstruction era, white Democrats regained power in southern states and reasserted white political supremacy through the passage of disfranchising legislation and constitutional amendments, as well as Jim Crow laws, including racial segregation. States followed this with more stringent laws classifying more persons as black based on traceable or any ancestry. For example, in 1822 Virginia, a person was considered legally white with up to one-fourth African ancestry (equivalent to one grandparent). Under its Racial Integrity Act of 1924, Virginia defined as black a person with any known African ancestry, no matter how many generations in the past. It also established a binary classification system for vital records, assigning persons to white or black categories (the latter was essentially all other, into which Native Americans were included.)