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%.
Some, including the bulk of the Cherokee Nation, have abandoned the strict criteria of blood quantum in favour of simple ancestry, based on rolls drawn up by a congressional commission in 1893.
It's a move sometimes derided by other Native Americans, who say the Cherokee have thrown the doors open so wide that practically anyone can claim membership.
Heading towards oblivion
For many, particularly the older generation, the issue of blood can be a matter of pride - a simple, incontrovertible measure of Indian identity.
But since Native Americans intermarry more than practically any other population in the nation, among the different tribes and with non-Native Americans, simple mathematics tells you that blood quantum is also a route towards oblivion.
It's also fiendishly complicated.
When I met Scott Davis at home in Bismarck in North Dakota to discuss the system, he brandished a piece of paper covered in fractions representing the blood quantum of his family.
Scott, executive director of the North Dakota Indian Affairs Commission, is 44% Lakota Sioux, with French and Chippewa blood.
His wife, Lorraine, is 36% Dakota Sioux, from the Sisseton-Wahpeton tribe.
When their daughters were born, his own Standing Rock tribe, which requires 25% blood quantum, did not recognise his wife's blood.
As a result, the girls were enrolled with the Sisseton-Wahpeton tribe, which had the same 25% requirement but accepted Scott's Dakota blood.
The girls enjoy 39.8% blood quantum.
Sometimes, it seems you have to be a quantum physicist to figure it out.
Faced with this gradual process of ever-diminishing fractions, tribes are looking at ways to prevent their gradual extinction.
These include reducing blood quantum requirements, the lineage route favoured by the Cherokee and the pooling of blood lines among tribes.
But others say it's time to do away with percentages and fractions altogether.
One possible solution is for tribes to adopt some form of citizenship act, with requirements to know your language and culture and to visit home.
For Jesse Taken Alive, councilman at large on the Standing Rock tribal government, it's important to stop thinking in terms of numbers.
"What's important is to focus on a way of life," he says.
Scott Davis would be only too happy.
"I don't believe in the system," Mr Davis says.
"In my heart and my mind, I'm full blood. I always will be full blood," he adds.