Richard Sander and Stuart Taylor wrote a book about this (Mismatch: How Affirmative Action Hurts Students It’s Intended to Help, and Why Universities Won’t Admit It)*, which they attribute to mismatch. As discussed here (http://www.sciencemag.org/careers/2012/12/how-not-attract-minorities-stem), before entering college blacks actually showed a bit higher interest in STEM. This only changed after enrollment:
*A decent overview and discussion is here: http://www.thecollegefix.com/post/24635/