Researchers at a university in Australia figured it out with the following test: They got two groups of people together: Some death metal fans, some not. Then they had them listen to one of two songs: Either Pharrell's "Happy", or a song called "Eaten" by the band Bloodbath. It's about cannibalism.
After each person listened to their song, they were shown two images, one to each eye. One of the images was violent, the other was non-threatening.
What normally happens is that when you're presented with two images like that, you tend to focus on the violent one more.
The thinking was that if death metal was really so bad, it would desensitize its listeners to violence, and they WOULDN'T focus more on the violent image. But that didn't happen.
The metalheads were just as focused on the violent image as everyone else. In other words: SCIENCE! And LONG LIVE DEATH METAL!!!