I have noticed, very recently, a few revisionists online claiming that the sacrifice of twins was a common practice among the Yoruba people during the Old Kingdom. I have said it before, and I will say it again: Yoruba people never engaged in the practice of sacrificing twins. But whoever is claiming otherwise is grossly misinformed; because, such an individual knows nothing about the Yoruba metaphysical thought process. Let us look at the Yoruba metaphysical thought process to put this argument to rest once and for all.
According to the Yoruba metaphysical thought process, our world is a chaotic market place (Aye loja) – chaos theory, created on the binary fortune of good and the misfortune of evil (Tibi tire la da ile aye) – binary theory, where only good character (IWA PELE) brings you closer to good fortune. The Yoruba understand the principle of harmonious binary opposites, and they regard twins as the perfect harmonious duality in human form (i.e., perfect “two truths”). There is nothing superficial, and I repeat, there is nothing superficial about this philosophical concept. It was this neat philosophical concept that led to the development of IFA or I Ching, which later morphed into binary code theory.
The Yoruba people look upon twins with such high veneration, much so, that twins are regarded as pseudo-orisha, Neteru or god in the Yoruba pantheon (Orisa Ibeji). In fact, the thesis that the Efik of Calabar were twin’s killers should be met with the antithesis of the Yoruba godly view of the twins, which is that twins are semi-deity in the Yoruba pantheon (i.e. Orisa Ibeji) - I strongly question Mary Slessor's account of twin's sacrifice among the Efik group of Calabar; this, I call a "missionary propaganda campaign." Anyway, how could twins be sacrificial and idolized at the same time? The answer is that twins are highly venerated in the Yoruba culture; so much so, that animals are often sacrificed to welcome their birth - twins were never sacrificed.
In conclusion, every culture of the world that sacrificed twins in antiquity did it for the same reason that many other cultures venerated them. That is, twins were regarded as perfect “two truths”, and thus were seen as the perfect harmonious duality in human form (Gerald Massey, natural genesis) - harmonious duality, they all believed, permeated our reality.