There are geniuses in every field, therefore every field is just as difficult intellectually? I don't follow your reasoning here. What sort of evidence would convince you that math/science are in fact harder for humans than history or art? (Edit: A somewhat different perspective I have is expressed by Feynman here: http://www.youtube.com/watch?v=NWjV0bNBPY4 The good men and women of a field are more important than the field, and you can probably find good men and women in every field if you look hard enough.)