Africa
On the other hand, in many African cultures, children are often given to adoptive families. By giving their child to another family, the birth family seeks to create enduring ties with the adoptive family. The placing family may receive another child from that family, or from another. In the same way as giving their child for adoption, the exchange of female relatives from one family to another, for marriage, is meant to create enduring connections and social solidarity among families and lineages. Among the Mbuti people, sister exchange is the common form of marriage. (see Mbuti people#Marriage customs)
Read more about this topic: Cultural Variations In Adoption