This is a slightly modified and shortened extract from a paper I gave at the Unlocking the Vikings 2014 conference.
One question I am almost invariably asked when I mention my thesis topic to people is, “How did berserkers go berserk?” People are fascinated by berserker pharmacology, berserker psychology and berserker physiology. Most commonly they want to know if berserkers really took magic mushrooms. When I inform them that we have no contemporary or literary evidence for the consumption of magic mushrooms as an aid in battle, they often move on to ask if they were just psychologically unbalanced, or had something physically wrong with them (like Paget’s Disease, as Byock has suggested for Egill Skallagrimsson).
There is a problem with this question: it assumes that berserkers actually did go berserk and never questions whether this is really the case. Some examples of warriors going berserk do occur in Old Norse literature. For example, At Vínheiðr, Egill Skallagrimsson’s brother Þórólfr appears to go berserk when he slings his shield on his back and strikes forward into the melee killing all around him as he goes:
‘Þórólfr gerðisk þá svá óðr, at hann kastaði skildinum á bak sér, en tók spjótit tveim höndum; hljóp hann þá fram ok hjó eða lagði til beggja handa; stukku menn þá frá tveggja vegna, en hann drap marga.’ (Thorolf became so enraged that he slung his shield on his back and took his spear in both hands. Then he ran forward and laid about him on both sides. Men all around him ran from him but he killed many), Egils saga, Chapter 53.
Likewise Haraldr harðraði at the Battle of Stamford Bridge appears to go berserk in a passage written in very similar style to that in Egils saga.
‘Þá varð Haraldr konungr Sigurðarson svá óðr at hann hljóp fram allt ór fylkingunni ok hjó báðum höndum. Helt þá hvártki við honum hjálmr né brynja. Þá stukku frá allir þeir, er næstir váru.’ (King Harald Sigurdsson became so enraged that he ran forward all the way out of his battle-line and laid about him on both sides. Neither helm nor mailcoat could withstand him. Then all those who were nearest ran away) Haralds saga Sigurðarsonar, Chapter 92.
The circumstances are similar to those under which modern soldiers have gone berserk, which suggests that the sagas’ authors were familiar with the idea that men might go berserk in battle. However, neither Þórólfr nor Haraldr is ever called a berserker. It is never stated that they undergo berserksgangr, and they do not engage in the activities normally associated with berserkers such as biting their shields or howling. The medieval saga authors did not treat them as berserkers and probably did not consider them to be berserkers.
Berserksgangr (usually translated as ‘berserker fit’ or ‘berserker fury’ although that is a different can of semantic worms that I shall save for another time) is signalled by shield-biting and howling. It usually occurs before a fight or battle and has been construed by most scholars as symptomatic of an altered state of mind: of going berserk. I dispute this, as the examples below show.
In Grettis saga (chapter 40), Snækollr begins biting his shield and howling even before dismounting from his horse, as he readies himself for battle with Grettir, an act that Grettir uses to good advantage. He kicks the base of the shield up into Snækollr’s mouth apparently breaking his jaw and then drags him from the horse and cuts off head. Snækollr was obviously not ready for battle and had certainly not gone berserk.
In Egils saga (chapter 64), the berserker Ljótr inn bleiki (Ljot the Pale) also bites his shield and howls at the start of the fight as he approaches the place for the duel. However, this berserksgangr is not an example of going berserk, because he then pauses to exchange words with Egill before taking a bit more time to get ready for the fight. Then, in the middle of the duel, Ljótr asks for a break, and he and Egill have a rest and a verbal exchange before continuing the fight which ends with the death of Ljótr. A berserk warrior is unlikely to have had the presence of mind to ask for a break.
Finally, to take one last example, the Viking or half-berserker Moldi in Svarfdæla saga (chapter 7) turns up with eleven men to discuss marrying jarl Herröðr’s daughter in a variant of the berserk suitor motif that Blaney identified as the most common usage of berserkers in sagas. He and his men enter the hall, wade through the fire and bite their shields. Then, instead of attacking in a frenzy as one might expect from the shield-biting motif, Moldi greets the jarl well and is invited to take a seat. Moldi’s suit does not end well, because he is killed in a duel as a result of the exchange regarding the jarl’s daughter, but he had certainly not gone berserk when he entered the hall, because he was sufficiently in control to stop and have a chat.
These examples show that the authors of these sagas did not consider berserkers to have lost all control. Given that berserksgangr only occurs at the start of combat, and not during the battle, it seems likely that the shield-biting and howling that are part of it were a form of posturing intended to help the berserker motivate himself for battle, while also intimidating his foes. One might compare it to the haka performed by the All Blacks before a rugby match which has all the hallmarks of berserksgangr with grimacing and posturing to put the opponents off while building team spirit and morale.
So, to answer the question in the title, berserkers in Old Norse literature did not generally go berserk. By extension, I would suggest that Viking berserkers did not go berserk either. Those elements of their activities that are remembered in Old Norse literature do not indicate a berserk state. Going berserk is likely to have been more of a hindrance than a help in a period when warfare relied upon maintaining an unbroken wall of shields. It is entirely possible, if not probable, that some warriors went berserk in any army of the period (not just Viking armies), because of the stress of warfare, but the evidence does not support habitual berserk states for Viking berserkers.