Among the Yoruba people of West Africa, an ibeji is a doll made to house the soul of a dead twin.

The Yoruba, who have the highest rate of twinning in the world (one set of twins for every 16-22 births, about three times the rate of North American or European twinning), believe that twins are sacred*. If one member of a set of twins dies, a wooden doll is carved about six to ten inches high, with the face of the family lineage mask. The doll is then treated as if it were the dead twin, still alive. Because twins are believed to share a single soul, the death of one is thought to cause the death of the survivor as well. The care and tending of an ibeji keeps the dead twin's soul in this world and ensures the survival of his sibling.

Though ibeji were originally carved wooden dolls, today they may also be plastic dolls or photographs. Parents often tend ibeji even if both twins have died. A recent estimate placed the number of ibeji at about half a million, belonging to about ten million Yoruba, so the practice is still very much alive.

* According to Yoruba mythology, twins are really colobus monkeys (who regularly give birth to twins) who have made a deal with the gods to be born as human. One of them is human, the other a spirit. Since you don't know which is which, you treat both of them as sacred.