Polygyny, a male marrying several females, is now rare except for Africa and especially Western Africa. Why would it be so prevalent in West Africa? To sustain polygyny, one needs an unbalanced sex-ratio, which is not the case there.
John Dalton and Tin Cheuk Leung claim that this is just a matter of very persistent institutions. Indeed, the sex-ratio used to be unbalanced for extensive periods in West Africa, and in a more pronounced and persistent way than anywhere else, due to the slave trade. Indeed, it took away many males from the region through the actual forced emigration, but also because of the many tribal wars associated with slave capture raids (which Dalton and Leung do not take into account).