What is 'blood quantum'?
Blood quantum might sound like an action movie, but to the country's Native Americans it's all about identity.
First introduced in colonial Virginia in the early 18th Century as a means of restricting the rights of anyone deemed to be more than 50% Native American, the term only became widespread after the Indian Reorganization Act of 1934.
By then, it had become a mechanism for calculating the amount of federal benefits a tribe could expect to receive, based on its population.
Over time, different tribes have adopted different levels of blood quantum.
Florida's Miccosukee Tribe is among a handful which require members to have 50% tribal blood - or the equivalent of one full blood parent.
At the other end of the spectrum, some Cherokee and Apache tribes require just 6.25% blood quantum. The largest number of tribes, however, require 25% or 12.5%.
Paul Adams explains.