Well, the authors get in too much detail of what catharsis is but their main focus is catharsis in collective rituals. Or in the case of heavy metal, a gig.
They didn't just interview fans, artists, and producers. They went a step further by attending metal gigs (i.e. the collective rituals!), collecting photos from the heavy metal scene, listening to a number of records from different metal genres (poor them!), reading metal magazines, going online to metal fansites, and downloading (legally? I may ask!) lyrics from the internet. So you see how they got my respect in the first place.
They start of with what they call the disempowered self. Meaning that their participating metal fans came from unhappy childhoods with feelings of loneliness and rejection from their peers due to their musical tastes. They move on to the collective un/conscious (e.g. stories, myths and symbols). The participants all seemed to have something in common: fondness of The Lord of the Rings trilogy and the idea that even the smallest person can change the world as well as an openness for accepting different sorts of people.
Then the authors move on to describe the ritual performance within heavy metal. This includes satanic symbols on clothing which indicate the desire for power. They also cite the lyrical themes representing social frustration through examples of mythical legends etc. Then on to emotional/cognitive balance, where the emotional experience of the gig is focused on anger and distancing. Here participants describe the nature of the mosh pit, stage-diving, and all the associated acts during a gig.
Finally, the authors describe rejuvenation where participants reported feelings of emotional relief and re-energizing, enlightenment, and moral rightness.
As you can see the author's built up a theory of catharsis within the heavy metal culture and specifically the live gig environment. Despite the use of fancy and scientific words here, one thing someone can take from this is that heavy metal fans "use" this gig environment as a punchball to release their day-to day frustrations and reach this said catharsis. From a psychological perspective I would add that heavy metal music and specifically a heavy metal gig may serve as a coping mechanism. It is really the one place you won't be judged, where you are a team (even when you get the odd punch in the face at the pit!), within a secure and friendly environment of sameness and differentiation at the same time. Now, I respect the authors try to dress this simple truth in a more philosophical attire but still I prefer my simple metal t-shirt!
P.S. The authors put a footnote to describe James Hetfield: "James Hetfield is the lead singer of the world’s most financially successful heavy metal music group, Metallica". Heh.
Headbanging as Resistance or Refuge: A Cathartic Account by Paul Henry & Marylouise Caldwell (2007). Abstract available here.
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