Researchers have suggested that Vikings may have originally started sailing and raiding due to a need to seek out women from foreign lands.  The concept was expressed in the 11th century by historian Dudo of Saint-Quentin in his semi imaginary History of The Normans.  Rich and powerful Viking men tended to have many wives and concubines; these polygynous relationships may have led to a shortage of eligible women for the average Viking male. Due to this, the average Viking man could have been forced to perform riskier actions to gain wealth and power to be able to find suitable women.  Viking men would often buy or capture women and make them into their wives or concubines.  Polygynous marriage increases male-male competition in society because it creates a pool of unmarried men who are willing to engage in risky status-elevating and sex seeking behaviors.  The Annals of Ulster states that in 821 the Vikings plundered an Irish village and "carried off a great number of women into captivity".