I'm mixed and must admit I find this an interesting rule
A lot also comes from one's upbringing. Mixed people often have an implicit "choice" to make and I've always felt the issue of that choice depends A LOT of where/with who we grow up and what we're exposed to. Culture, people, history, etc. If you grow up in a majority white environment, most likely you'll navigate that way...unless you're proactive in connecting with your black side. If you grow up in a majority black environment you usually stick with it, at least that's what I've seen with the limited sample of other mixed people I've met over the years.