Hello, I'm writing an article about Viking Berserkers and I've scoured the net for the answer to my question and I can't seem to find one. Mainly my question is, is there any criteria Vikings had for choosing who was a berserker? Did they get chosen by a Jarl, or inherit it, or just decide one day "Hey I'm going to do that"? Or something else? Thanks so much for your time.
Well, for one, we don't know if berserkír, as described in the sagas and popular culture, existed. What evidence we do have for them shows them not as frothing lunatics, but rather as highly skilled, very independent-minded warriors, usually employed by a king as part of his elite.
In literature, berserkír fulfil a couple of roles: Either as the Monstrous or Other, as is the case with the Kveld-Ulf, Grím, and Egill in Egils saga, or are highly skilled and occasionally magical warriors roaming the countryside for wealth.
If you're interested in some theories behind what could have caused the 'berserk' state, Lily Geraty just wrote her MA thesis on combat trauma as a possibility, and it's available free on Skemman (PDF warning)