From the US Department of the Interior: https://www.doi.gov/tribes/enrollment:
Tribal enrollment criteria are set forth in tribal constitutions, articles of incorporation or ordinances. The criterion varies from tribe to tribe, so uniform membership requirements do not exist.
Two common requirements for membership are lineal decendents from someone named on the tribe's base roll or relationship to a tribal member who descended from someone named on the base roll. (A "base roll" is the original list of members as designated in a tribal constitution or other document specifying enrollment criteria.) Other conditions such as tribal blood quantum, tribal residency, or continued contact with the tribe are common.
How to Apply:
After you have completed your genealogical research, documented your ancestry, and determined the tribe with which your ancestor was affiliated, you are ready to contact the tribe directly to obtain the criteria for membership.
Rarely is the BIA involved in enrollment and membership. Each tribe determines whether an individual is eligible for membership. Each tribe maintains it's own enrollment records and records about past members.
The Tribal Leaders Directory that is published by the Bureau of Indian Affairs lists all 562 federally recognized American Indian Tribes and Alaska Natives. The electronic, map based, interactive directory also provides information about each BIA region and agency that provides services to a specific tribe.
Guide to tracing ancestry: https://www.bia.gov/sites/bia.gov/files/assets/foia/ois/pdf/idc-002619.pdf
Certificate Degree of Indian Blood Application & Instruction: https://www.bia.gov/sites/bia.gov/files/assets/foia/raca/pdf/idc1-029262.pdf
National Archives Resources for Genealogists: https://www.archives.gov/research/genealogy
Atlas of historical county boundaries: http://publications.newberry.org/ahcbp/
Access Genealogy: https://www.accessgenealogy.com/america
This site provides an array of historical records and resources with regards to sociology. They also offer articles on specific tribes and historical events.
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