Okay, but why do AA little girls have hair that is dry, breaking, and difficult to manage, even before getting chemical treatments or using heat?
I agree that parents should not relax their children's hair at an early age, but people forget to acknowledge WHY parents do that - they do it because their child's natural hair is already tough to manage or broken. My niece is a prime example - her hair is thinning & it grows sooo slowly, even though she is natural, her mom DCs and moisturizes constantly, and she wears protective styles all the time.
As far as other races, they DO use chemicals and heat. Every non-black woman I knows dyes her hair and uses heat appliances. They may get splits and dry hair, but it doesn't stop retaining and visibly getting longer.
I really can't agree with this enough, when we were children most of the AA girls mothers/guardians used mostly the same products to comb and dress our hair, but not all of us had long hair.
Unfortunately, even though I have been on this forum for over a year I debate with myself (because no one else wants to talk hair growth with me IRL) constantly if genetics doesn't also play a part in my hair's personality (length, texture, etc). If not then why has my always been b/n SL and APL, why even grow that long?
I do know now how to baby my hair and so now I am at BSL and beyond but honestly if I had never found this site I would have thought that AA women having long hair was just the luck of the draw, solely determined by genes.
To answer the OP: I have known fewer than ten AA women with BSL hair and longer is an extreme rarity, only on this board does it seem almost commonplace. But when I see polls on this site inquiring what everyone's length is the number #1 length seems to always be shoulder length, not even APL. I now believ that our treatment of our hair (relaxers, heat, combing, using grease, not washing enough, lack of moisture) is what keeps our hair shorter on average than other races.
I agree that our hair breaks faster or as fast as it grows which is why it so difficult for us to retain length, but I also think that genes play a role. Because like many a poster on this thread has mentioned not all non-black women have thick, long flowing locks either.
I live in NYC and there are plenty of black women here, but I have to say growing up I didn't see alot of women with SL, APL or BSL hair. The girls in school used to coo over my hair and it was just past shoulder length, no BSL or WL for me ever.
But now when I walk the streets of NYC I see many more women with BSL hair and to be honest, like I said before, before I came to this board I would have just thought that either your hair grows that long or it doesn't. I would have never attempted to grow my hair longer.
That was a bit long winded, Sorry but this is just a thought that often runs thru my head
LOL! 
erplexed
). It seems to be a given that that is true; I guess people would rather address why that is the case. I think the fact that they address the why without even bothering to address the if is evidence that people know that, by and large, black women's hair is shorter than most everyone else's.
But what I am saying to you, is that I think the long haired gene, or short haired gene or whatever it is, is passed more from family to family, than being a racial characteristic!
