Blue Exorcist: Nature or Nurture?

It is a question of humanity if we are determined by our nature or by those who nurture us. If you ask a room full of people this question, you will get a variety of different answers. So how are we suppose to know? An anime series (that was originally a manga) called Blue Exorcist brings this question into focus as the lead, Rin, and his brother discover that they are the sons of Satan.