I'm reading Neil Oliver's memorably titled, "Vikings, a History," and he argues against the idea that overpopulation and fixed socioeconomic hierarchy drove young men away from home. To quote, "They simply looked out from their own fjords and bays, saw how well the neighbors were doing in thier marketplaces, and set about claiming as much as possible for themselves."
It seems plausible enough to pass the smell test. But the author's only supporting evidence is a shallow overview of continental politics. Can anyone shed more light on how historians would assess one theory vs the other?
It is true that there is almost no consensus even among the researchers on the single cause-effect on the beginning of the Viking Age, but at least I also argue against the classic over-population hypothesis.
As I discussed in Did the Nordic countries use to have a comparatively larger population back at the time of the vikings? If not how were they so often able to raid Britain?, the estimated population of Viking Age Scandinavia was less than half of the maximum estimated population of pre-modern Scandinavia just before the Black Death. This estimation is mainly based on the estimation of how much arable land was utilized, roughly calculated by place name evidence of the provenance of individual farms.
In short, there was still some land to spare, but some Scandinavians chose to went out of their homeland in Viking Age Scandinavia. As for the agricultural technology, there was no clear revolutionary rupture in Scandinavia between the Viking Age and the Middle Ages, so the Vikings could utilize these lands to cultivate if they would really wanted to do so.
It is also worth noting that the climate of Viking Age Scandinavia that belonged to 'medieval warmer period' is generally regarded rather as favorable than that of the High Middle Ages, especially after the middle of the 13th century, in the beginning of the Little Ice Age.
The maximum number of the peasant navy levy in medieval Norwegian law (336 ships for the whole Norway) that I alluded in the linked answer also suggests that the number of ships of the raiders mentioned in the account of the victims in the British Isles as well as in the Continent is also probably exaggerated, at least to some extent, as I and others explained before in How large would Viking raiding parties be?.
In fact, the identification of 'large-scale' barbarian migration as 'driven by the over-population' has been a long-aged literary cliche, dating back further to (Late) Antiquity (Wolfram 1994). A Christian chronicler, Dudo of St. Quentin, borrowed this cliche to glorify the 'exodus' of the ancestor of the Normans in Normandy, then this explanation became widely known as a possible cause of the Viking immigrants in general, especially in modern times.
As a variant of this tradition, some scholars even now proposes that the gender imbalance among the Scandinavians (i.e. over-population of young, male), due to the practice of gendered infanticide (selective female infant killing), could be a contributing factor of the Viking Ages (Barrett 2008), but I suppose this alternative does not sound so convicting since the practice of infanticide did certainly not begin suddenly in the 7th or 8th century Scandinavia.
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