Intelligent kids don't have a lot of friends because they don't care for being popular in school, so they too don't hang out as a "cool" kid, he have a lot time without a thing to do. They pass the day in computer.
They listen a lot of music, different than the others music, they want a music that isn't for popular persons, only for sad persons, so he find punk rock, later, heavy metal, later, he find the underground metal/punk.
Now, listening to it, he discover how boring his life is, so he start do new things, he learn play guitar, he meet other metalheads, he start skateboarding, he is happy, he find a totally new world.
And now I'm here today.
I think that is that happen, but I too think that isn't metal that makes the kid more intelligent, but himself had born intelligent, metal only free him of the normal life mediocrity.