Webster's notwithstanding, according to the US Congress, Office of Technology Assessment,1 generally the power to define their membership resides with the tribes, and this power has been repeatedly recognized by federal courts.2-4 For the most part, tribes have required some level of Indian blood (roman mine) of the specific tribe for membership. Most require at least a one-eighth blood quantum.5 I suppose blood is thicker than science.
DeAngelis C. Bloody Insensitive-Reply. Arch Pediatr Adolesc Med. 1996;150(5):560. doi:10.1001/archpedi.1996.02170300114033