If the goal is to get the most and best answers, and you're not engaging all demographics, then you can get more and better answers by doing that.
1) People genuinely wishing to share their knowledge
2) People that are chasing gamified elements of the reputation system
3) People that belong somewhere between #1 & #2
I dont see anything that prevents any demographic from participating as long as fall into one of these 3 categories. And i dont see how being of a particular gender or race or ethnicity precludes one from sharing their knowledge on StackOverflow