It is a very difficult question, and I don't have an easy answer. I have taken the best advice I can get, and the advice that I have been given is that it's a matter of how people self-identify and how they feel culturally connected. Some people look very different visibly but may not feel any connection with their community or their cultural group, whereas others may. We do it on that basis.
On March 31st, 2009. See this statement in context.