Any other ethnic or religious group in the world need only declare their existence. Only the American Indian is required to document genealogy to the beginning of time and blood quantum to show how much real "Indian" they are.
Intrigued by the concept of "blood quantum," I did further investigation, and I learned something new.
When people set percentage quotas for blood in order to determine membership to an ethnic and/or religious group, the restrictions draw arbitrary boundaries between who does and doesn't belong. A person with a greater blood quantum of Native American ancestry is considered a more authentic and worthy Native American than anyone else. Thus, people who have smaller blood quanta are considered less Native American and less worthy of recognition and benefits. Also considered to be less worthy and less Native American are those who, for whatever reasons, cannot document their ancestry to the satisfaction of the regulations. Furthermore, blood quanta fight a losing battle because they either stay the same throughout the generations or become lower and lower, meaning that smaller and smaller numbers of Native Americans will qualify for benefits and recognition if the current blood quanta stay in place. Though created to solidify and preserve a specific group, the concept of blood quanta actually weakens, divides and damages the group. Nokwisa Yona's 2001 editorial on a proposed Vermont bill, H. 809, that would make DNA testing a means of determining "Indianness," elaborates on the racism of blood quanta.
Let's say that I think I might have some Native American ancestry. I got this idea today because my paternal grandfather's ancestors have lived in the New England/upstate New York area for ~300 years, after coming from England. Also my paternal grandmother's ancestors have lived in New York/Vermont/Quebec for ~200 years, after coming from France. These two branches of my family cover both the geographic areas of the Abenaki settlements and the periods of cultural interchange between immigrants and natives. Therefore, it seems possible to me that some of my ancestors at some time intermarried with some Native Americans, and I may have some Native American ancestry somewhere around in the roots of my family tree. To someone in my situation, blood quanta say, "Well, you can do all the genealogy and dig up all the circumstantial evidence you want, but it won't really count because, if you do find out that you have some Native American blood, it's not pure enough for us."
On the subject of Native American ancestry and identity, I'm caught between the huge excitement of another genealogical journey [focusing on my maternal grandmother's French Canadian ancestry] and the nasty tendency of non-Native Americans to romanticize and appropriate Native American identities by writing melodramatic memoirs full of lies. [Link to David Treuer's insightful commentary on why the faux memoirists choose Indian identities. Answer: Because of the romance of suffering.]