Great question! I think some hip hop has a negative effect on people because of the language that is being rapped, chanted, sung or whatever. The music may be good but the lyrics are what is most damaging. If you have these words on your mind, when you react, you react with the lyrics you have learned.
As human beings, words are what we use to communicate and when you use words, you are feeding your spiritual man. This is why God wants us to think on good things! (Philippians 4:8)
If we are rapping educational facts then maybe our children would have more information in their heads. I would suggest we use these methods for postive and stop accepting negative.