Around 800, men from Scandinavia start crossing the North Sea in growing numbers to plunder monasteries. Why then, and why them? One answer was already on offer around the year 1000, and it sounds surprisingly modern. Since then overpopulation, a shortage of women, the sail, silver, Charlemagne, a climate catastrophe and the pursuit of status have joined the list. This article holds all seven explanations to the same three standards and shows what bears weight and what does not. No single cause survives – but neither does the idea that any list will do.
The oldest explanation came from a man who had witnessed no raid. Dudo of Saint-Quentin wrote his history of the dukes of Normandy between the late tenth and early eleventh centuries, some two hundred years after the first expeditions, for the very family descended from such conquerors. His Scandinavians, whom he calls “Dacians” in the learned fashion, live without restraint, take many women, father too many children and send the surplus abroad by lot.
He namque gentes petulanti nimium luxu exardescentes feminasque quamplurimas singulari turpitudine stuprantes commiscendo … soboles innumeras … generant. […] atque terram quam incolunt habitare non sufficientes, collecta sorte multitudine pubescentium … in externa regna extruduntur nationum, ut acquirant sibi praeliando regna.
“For these peoples, inflamed by excessive licence, defile very many women with singular shamelessness and beget … countless offspring. […] And since they cannot find room in the land they inhabit, a multitude of youths is chosen by lot and thrust out into the realms of foreign nations, so that they may win kingdoms for themselves by fighting.”Dudo of Saint-Quentin, Historia Normannorum, capitulum 2 in Felice Lifshitz’s numbering (Berlin manuscript, fols 8v–12v); Latin wording read through a retrieval tool from her transcription (Fordham Sourcebook), abridged, one corrupt spot there omitted, not checked against a printed edition; English version my own translation
Read closely, this already contains two later research theories: too many people for too little land, and men who tie many women to themselves. It still fails as a factual report. Dudo is writing a moralising origin story and reaches for a pattern that suits the image of barbarians: they are unrestrained, therefore numerous, therefore dangerous. He proves nothing. He does show how old the temptation is to pin the whole phenomenon on one reason.
“Why did the Viking Age begin?” bundles several questions together. Voyages across the Baltic were nothing new: the boat graves at Salme on Saaremaa hold around forty slain men, probably from what is now Sweden, usually placed around 750 or somewhat earlier. The excavator Claus Feveile dates the regular plots of the trading site of Ribe by tree rings to 704–710, with a looser first use also in the early eighth century. Scandinavia was on the move and connected well before Lindisfarne was raided in 793. Why that date is only a convention is covered in our article on the raid and need not be repeated here.
What is new is something else: sea raiding across the open North Sea against targets in the Christian West, on a rising curve. The archaeologist James Barrett, whose 2008 paper “What caused the Viking Age?” remains the starting point of the debate, warns against reading history backwards. The many reports from the mid-ninth century onward concern armies, overwintering and land-taking. The first overwinterings he lists fall in 841 in Ireland, 850/51 in England and 852/53 in Frankia. Whatever explains the conquest of England need not explain why a few ships set out decades before. The crews who landed at Portland under King Beorhtric of Wessex (786–802) could have known nothing of what followed.
That allows every explanation to be treated alike. First, time: does it fit the late eighth century, or was the factor at work much earlier or only much later? Second, place: scholars look for the early attackers in western Scandinavia, above all western Norway, and their targets were rural monasteries. That origin is not certain, since the sources mostly speak only of heathens. A theory that depends on Denmark or the great trading towns has a problem here. Third, evidence: are there data from the period in question, or do they come from texts written centuries later or far away?
One further distinction helps, and popular accounts often skip it. A precondition makes something possible, a pressure or incentive makes it likely, a trigger sets it going. Ships may be indispensable without moving anyone to leave.
Land shortage long seemed the obvious answer, especially in narrow western Norway: little arable soil, many sons, so off they went. Barrett does not deny that Europe’s population grew in the early Middle Ages. He doubts the inference. His strongest argument comes from the ground itself: forest clearance in many parts of Scandinavia advanced during the Viking Age rather than before it, and in Norway may even be typical of its end. People who can still open up land at home are not backed against a wall.
Then there is the sequence. Barrett finds no reliable evidence of Scandinavian settlement in Atlantic Scotland before the mid-ninth century and regards earlier radiocarbon dates from Norwick on Shetland and Pool on Sanday as skewed. Iceland’s oldest settlement traces follow only in the 870s, almost a century after Lindisfarne, Greenland at the end of the tenth century. Hungry emigrants would hardly raid first and settle three generations later. Nor did expansion move as an even wave. It jumped along personal connections; Barrett speaks of “leap-frog” patterns, skipping whole regions in between.
The topic is not quite closed. The archaeologist Steven Ashby points to a variant by Dagfinn Skre in which newly created freemen farm additional land, which would make access to land and standing the problem, not headcount. How many people lived in Scandinavia around 790 cannot be established with confidence. The useful argument is therefore not about totals but about distribution, which leads straight to the next theory.
Barrett himself proposes a demographic reason of another kind: not too many people, but a surplus of young men. They needed wealth to marry and found a household, and they fetched it from overseas. For the idea of a “youth bulge” he draws on Gunnar Heinsohn. As a possible mechanism he names selective killing of newborn girls. His archaeological pointer: loot from the British Isles often lies in women’s graves in western Norway, which looks like bride-wealth.
In 2017 Ben Raffield, Neil Price and Mark Collard rebuilt this in Evolution and Human Behavior. In their version the male surplus arises not from births but in the marriage market. Polygyny, meaning several wives for one man, and concubinage, combined with growing inequality, concentrated women among wealthy men. The technical term is “operational sex ratio”, the ratio of men and women ready to mate at a given time. What remained was a pool of unmarried men more willing to take risks. For leaders and followers alike, raiding became a bargain that served both. A 2015 conference paper on the same subject still names the killing of girls as a second mechanism. In the 2017 article, as far as the retrieval tool rendered the text, it no longer serves as evidence.
Against the three standards, this is strong in logic and weaker in data. The evidence for plural marriage comes from Adam of Bremen in the eleventh century, from Ibn Fadlan’s account of the Rus in the tenth, and from Icelandic laws and sagas written down long after the conversion. The authors say so themselves and call their approach probabilistic, not deterministic: it raises a likelihood and compels nothing. They offer no measured sex ratio for western Norway around 790, and I know of none elsewhere. That most of the dead at Salme, Ridgeway Hill and Oxford are young men fits the picture, but it shows only who fought, not why he is supposed to have been unmarried.
The trail of infanticide is thinner still. Ashby calls the indications “rather circumstantial”: Gunnlaug’s saga from the thirteenth century, few identifiable female graves and an Arabic travel account from Hedeby. That account, preserved in late transmission, speaks in the version our article gives of children in general being thrown into the sea, not of girls specifically, whereas Ashby cites it as a hint at the sacrifice of girls, and it may follow a cliché about heathens. Few female graves, for their part, can also mean different burial customs.
The picture is seductive: the North acquires the sail and England is suddenly within reach. In fact the date remains open. Barrett reckons that sails may have been adopted several centuries before the Viking Age and notes that long boat journeys were already familiar: the weapon sacrifices at Nydam I in the fourth century presuppose them. The archaeologist Christer Westerdahl, by contrast, places the transition to sailing in the eighth to tenth centuries. The Kvalsund boat offers no clear proof of a rig, and masted ships appear surprisingly late among the finds: the Oseberg ship, built around 820, still carries its mast in a rather weak step. Jüri Peets suspects a mast for the larger Salme boat; that remains unproven.
Even a perfectly timed sail would explain only the how. A voyage from Norway to Ireland took under two weeks by Barrett’s estimate, and skills such as clinker shipbuilding were in place. But a ship says nothing about who builds it, who rows it and what for. A large sail also took a great deal of work; Westerdahl suspects the strips were woven across many households. That makes the sail more a sign of organised power than its cause. Barrett also considers something other than new technology: a late-eighth-century change in how existing ships were used, such as tacking or sailing round the clock. That is conjecture.
How Scandinavians came to prize silver is set out in our article on Viking trade. For the question of causes only one thing counts: was demand the reason? The timing is striking. According to Barrett, Abbasid silver reaches north-west Russia and the Baltic in the last two decades of the eighth century; a coin hoard near Staraya Ladoga contains issues from 749 to 786. The archaeologist Władysław Duczko spoke of a “silver fever” that may have spread westward.
The place test applies the brakes. The North Sea trading sites boomed in the “long eighth century” but were attacked chiefly in the 830s and 840s. The first targets were rural monasteries, the early raiders probably country people. Barrett also assumes that the loot stayed in Scandinavia instead of being sold on for Arabic silver. Søren Sindbæk objects to the rural reading: he understands urbanism as a network and sees the roots of the Viking world precisely there. Ashby adds that long-distance networks for Arctic resources appear to be older than the Viking Age. This dispute is not over.
The milder version is hardly contested: trade supplied knowledge. The Anglo-Saxon Chronicle says only that the reeve at Portland did not know who the strangers were. Æthelweard, writing in the late tenth century, is the first to say he took them for merchants. That is no eyewitness statement, but it fits a world in which such contact could seem routine. How the attackers knew there was precious metal on Lindisfarne or Iona, though, is recorded nowhere. Barrett speculates about a meeting of Norwegians and Irish on the Faroes or the Northern Isles, and himself calls it hypothetical. All that is firm is that the monasteries had more to offer than the thinly settled islands along the way.
A second commodity is easily overlooked: people. Raffield and colleagues explicitly count captives among what the men hoped to seize, and Skre holds a “slavery hypothesis” of his own, which Barrett classes as controversial. How much slavery contributed to the beginning cannot be quantified.
From 772 Charlemagne waged war on the Saxons. The story of Widukind, the Irminsul and Verden is told elsewhere. What matters for the North are a few terse entries in the Royal Frankish Annals. In 777 Widukind fled “to the regions of Nordmannia”; the revised version names the Danish king Sigifrid as his host. In 782 Sigifrid’s envoys came to Charles, and in 798 a Frankish envoy travelled to him. A Christian great power was moving up to the border, and the two sides were talking.
In 1993 the Norwegian archaeologist Bjørn Myhre put forward a broader model: influence and pressure from continental centres sped up power-building in the North, and the expansive Christian realms provoked a backlash among pagan elites. Barrett, through whom I know Myhre’s model, ranks it among the more controversial ones, offered heuristically. Interpreted modestly, this finds support. In 804 the Danish king Godfred stands with fleet and cavalry at Sliesthorp on the Saxon border. In 808 he destroys the trading place of Reric, moves its merchants to Sliesthorp and has the frontier secured by a rampart, which is often linked to the Danevirke. In 810 the emperor at Aachen hears that a fleet of 200 ships from “Nordmannia” has ravaged Frisia and extorted 100 pounds of silver in tribute. Whether the number of ships is right cannot be checked.
The problem again lies with place. Godfred is a Danish actor on the southern frontier, and he acts when the western raids are already under way. The early attackers of the British Isles, as Ashby notes, probably came from western Scandinavia, far from the Saxons. No source shows a Hordaland farmer plundering Lindisfarne in revenge for Verden. No contemporary text that I know of names the Saxon wars as a motive. Barrett also argues against a simple “weak neighbours” model: Offa’s Mercia and Charles’s empire were strong when the raids began. What holds up is the broader version, which sees power-building in the North as an answer to a mighty model and a threatening neighbour. The revenge formula is a modern overstatement.
The climate crisis of around 536 has become a fixture of the popular picture. According to the abstract of their 2012 paper, Bo Gräslund and Neil Price see good evidence for a climatic downturn and widespread disruption of settlement in the North, and they suspect an echo in poetry and sculpture. The abstract does not mention the Viking Age. What is documented for the North and where the figures wobble is shown in our article on Fimbulwinter: graves are not a census, and the study quoted there for southern Norway, in which burials fall by more than three quarters, measures graves in the first place.
As a direct cause, 536 fails the time test, for around ten generations lie between. Only a long chain deserves thought. Daniel Löwenborg reads the evidence from the Mälaren valley as a crisis after which rights to abandoned land were renegotiated and a small elite gained. Such inequality could later produce exactly the imbalance Raffield presupposes. Every link in that chain is interpretation, and none of the texts I have read shows that it runs through to 793 for western Norway. It also sits awkwardly with the overpopulation idea, since a shrunken land with abandoned farms first had to fill up again. That does not rule out renewed growth, but the figures are missing.
Writing in Archaeological Dialogues in 2015, Ashby targets a weak spot: counting only silver and bride-gifts does not explain what came back. Many pieces from the British Isles were not melted down but reworked and worn by women as jewellery. The Vale of York hoard held a mid-ninth-century Carolingian cup that was kept for over half a century; for what purpose, Ashby admits, we do not know. Such things, on his reading, told of voyages into the unknown. He draws on the anthropologist Mary Helms, for whom elites derive power from distance and from dealing with it. One of his most pointed sentences holds that the act of acquiring silver was as important as the silver itself.
This suits a time when rank was negotiable and leaders had to keep their followers content. Barrett names competition among chieftains and kings as a driving force; later come exiles like Harald Klak, and in the ninth century the fleets grow. Barrett counts three ships at Portland, 13 in 820, and two fleets of 60 in 837; the 200 ships of 810, as a Frankish report, do not fit that tidy series. The weakness of the prestige theory: it explains well why men went along, and poorly why just now. Warlike masculinity existed all over early medieval Europe. For that reason Barrett also ranks the much-quoted Norse fatalism as no more than a precondition. Ashby himself does not aim to replace existing models, only to qualify them with a more explicitly social element.
The three major papers end in remarkably similar ways. Barrett combines long-term developments with contingent events. Raffield and colleagues write that other factors were probably involved. Ashby considers the search for a single reason futile and speaks of push and pull factors in a catalysing environment. Anyone wanting to know what share falls to which factor gets no answer: these sources cannot establish that.
What was the main reason for the Viking Age? Scholars recognise no single one. Most often they combine young men’s search for wealth and standing, competition among leaders, seaworthy ships and news of rich, poorly protected targets.
Were there really too few women? That has not been measured. Raffield, Price and Collard argue that polygyny and concubinage could have concentrated women among the wealthy. The scarcity would then be one of the marriage market, not of total numbers.
Did the climate crisis of 536 create the Vikings? Not directly, as more than 250 years lie between. A long indirect effect through landholding and inequality is possible, but it remains interpretation.
Did the Vikings attack because Charlemagne subdued the Saxons? There is no direct evidence for that link. Contacts and tensions between Franks and Danes are well documented, but the earliest push against the British Isles probably came from western Norway.
Why did they attack monasteries in particular? Precious metal lay there in often unprotected coastal places. How the first attackers knew this is not recorded; trade contacts are the obvious answer, though unproven.
The Lindisfarne Raid of 793 · Viking Trade · Why did the Vikings stop raiding? Six explanations put to the test · Widukind.
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