where recorded in regions where polygyny was practiced, but delayed by males. Enslavement of women 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, and these polygynous relationships might 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. The Annals of Ulster states that in 821 the Vikings plundered an Irish village and "carried off a great number of women into captivity". Polygyny in West Africa, the region of the world where the practice is most common, exists in a societal context where historical factors, such as the slave trade, and the local religion, Islam, interact creating a culture where polygyny is widespread. The slave trade's impact on the male-to-female sex ratio has been cited as a key factor in the high prevalence of polygynous practices in this region of Africa. Findings Of the 1,231 societies listed in the 1980 Ethnographic Atlas, 186 were found to be monogamous; 453 had occasional polygyny; 588 had more frequent polygyny; and 4 had polyandry. Some research that show that males living in polygynous marriages may live 12 percent longer. Polygyny may be practiced where there is a lower male:female ratio; this may result from male infants having increased mortality from infectious diseases. However, the natural sex ratio at birth is actually slightly biased in favour of males (the natural sex ratio is around 105 boys/100 girls at birth; generally in the range of 103–107); presently in some countries this is even more biased in favor of males due to sex selective abortion and female infanticide. Other research shows that polygyny is widely practiced where societies are destabilized, more violent, more likely to invade neighbors and more likely to fail. This has been attributed to the inequality factor of polygyny, where if the richest and most powerful 10 percent of males have four wives each, the bottom 30 percent of males