Blood quantum became central to federal policy in the late 1800s and early 1900s. Under the Dawes Act era, officials recorded fractions for each person on tribal rolls, often based on guesswork. The Indian Reorganization Act of 1934 defined Indian to include people of one-half or more Indian blood who were not members of a recognized tribe. The Bureau of Indian Affairs issues a Certificate of Degree of Indian Blood.
The system works in the opposite direction from the one-drop rule. Where any African ancestry made a person Black under Jim Crow laws, blood quantum requirements meant Native identity could be diluted away through intermarriage. Critics say it was designed to shrink the number of people the government owed treaty obligations to.
Tribes have the right to decide their own membership, which the Supreme Court affirmed in Santa Clara Pueblo v. Martinez in 1978. Some tribes require a minimum blood quantum, while others, such as the Cherokee Nation, enroll anyone who can trace descent to an ancestor on the Dawes Rolls. Many Native scholars warn that strict blood quantum rules could eventually shrink tribal citizenship.