Like so many studies, this one failed to ask the question, what caused which?
Polygyny was already a factor during the slave era--and the authors admit this. But what they don't envision is that a conquering tribe would have retained the captured women of the conquered tribe, whilst selling the men as slaves. This scenario turns the whole theory on its head: the sexual disparity in the slave population was not a cause of polygyny, but a result of it.
West African slaves were mostly sent to the New World, where buyers strongly preferred men capable of performing backbreaking tasks on plantations. By contrast, buyers in slave trades centered on the Indian Ocean and Red Sea were often looking for women who could work as domestic servants or concubines.Now, taking my approach, we see that polygyny in East Africa may have been stifled by a lack of women, the surplus of that sex having been depleted rather than augmented by the slave trade. So if anything was a result of slavery in Africa, it was monogamy. What native culture could not manage to effect--the suppression of polygyny--the outside force of slavery could.
Record-keeping by European slave traders shows a consistent pattern, Dalton and Leung found: Between 1545 and 1864, 66.4 percent of slaves sent to North America and the Caribbean from present-day Senegal and Gambia were men, as were 66.6 percent sent from Sierra Leone, 65.4 percent from the Gold Coast (now Ghana), and 65.4 percent from the Windward Coast (now Ivory Coast). Going a step further, Dalton and Leung looked at data on the slaves taken from specific ethnic groups and compared it with the percentage of women in those groups who today share husbands with other wives. (They controlled for factors such as education level and religion.) The researchers found that groups hit heavily by trans-Atlantic slavery were significantly more likely to have a high percentage of polygynous marriages.
As the Western world slides ever further away from monogamy, we wonder what it will take to reverse that slide, and from where such a powerful force may come.