It all depends on what type of metal head we are speaking about.
I've found that the more common metal head who listens to commercial bands such as Metallica, Slipknot, Iron Maiden, Cradle of Filth, Pantera etc. is usually of low-to-average intelligence, more or less listening to this type of music to fit into a pseudo-alternative crowd. Most commercial heavy metal culture (or my pals culture as Vikernes would call it) consists of less mentally stimulating (sometimes degenerating) activities like drinking, head banging, casual sex, and drug use. These attributes are more common to people with lower intelligence that need to find distractions in life to amuse themselves because they are otherwise bored shitless.
Your more underground metal head, one who listens to black and death metal in particular, I've found to be usually of substantial intelligence, more inclined to be interested in intellectual activities such as philosophy and writing. They are also more humble, usually shy, and resentful to the modern world of which encourages ignorance and stupidity.
What you base the measurement of intelligence on also becomes important, just because someone doesn't perform well in algebra doesn't mean they are of less intelligence then someone who can, instead I prefer to base it on their behaviour, ideas and creations.
