Trouble at home caused by wild and violent Vikings returning from raids may have led to the first criminal profiles, an expert has claimed.
Icelandic sagas from the Viking age contain detailed descriptions of individuals most people would choose to avoid. The poems focus on aspects of physical appearance now known to be associated with high testosterone levels and aggression, according to historian Dr Tarrin Wills.
He believes they served as warnings at a time when disorder and violence threatened Icelandic society.
They featured colourful characters such as Egill Skallagrimsson, who committed his first murder at the age of seven and "had a lifelong interest in homicide".
Egill is described as having the classic testosterone-driven characteristics of a wide forehead and face, bushy beard, broad shoulders and receding hairline. Shortly before he died he buried a hoard of treasure - and killed the two slaves who helped him hide it.
"A good Viking should be aggressive and dominant, he should go abroad, he should rape and pillage," said Dr Wills, from the University of Aberdeen. "But many of these guys end up back home having to settle down to what is basically farming and family life. The kinds of guys that are good at rape and pillage aren't very good husbands and farmers.
"This was a particular problem in Iceland because Icelanders, like the rest of Scandinavia, had a very sophisticated legal system but no central government, no way of enforcing the law."
Dr Wills told the journal Viking and Medieval Scandinavia that vikings got into fights for all sorts of reasons. Typical causes of conflict were stealing neighbours' land, arguing over horses, insulting poems and even ball games.
The academic began investigating the way testosterone-fuelled vikings behaved after reading an article about hormones and aggressive, driven city traders.
"I followed this lead by reviewing the scientific literature on physiological and behaviour traits linked to testosterone," he said. "The profiles seemed to describe patterns I was familiar with in early 13th-century Icelandic literature."