Originally Posted by
Ecurb
All human societies about which we know have incest taboos (in anthropology this is the sole universal human rule). The precise nature of the rule varies, although we know of no societies in which parents are permitted to have sex with their own children. Brother and sister incest has been allowed only in rare cases, usually among royalty (Egyptian pharoahs sometimes married siblings, their only divine equals).
There have been many "explanations" of the universality of these rules, including psychological, biological, political and economic theories. The most common, biological explanation (greater likelihood of recessive traits leading to deformity) is highly dubious, mainly because it is falsified by the very common marriage rule that although one cannot marry one's "parallel cousin", one must marry one's "cross cousin". A cross cousin is one's mother's brother's or father's sister's child; a parallel cousin is one's father's brother's or mother's sister's child. Although in English we call all these relative's "cousins", in many languages they are referred to by distinct kinship terms. Also, in small, tribal societies, everyone is a relative to everyone else, so you must marry some sort of relative.
Freud was fascinated by the incest taboo, and wrote a book about it ("Totem and Taboo"). It's an entertaining book, although anthropologists pooh-pooh the ethnography and the wild speculations. "Exogamy" ("marrying out") certainly has economic and political advantages. After all, if you married your sister, your child would have only two grandparents to help him out, instead of four. In general, "cross cousins" are members of different clans, so marrying them builds economic and political alliances that would not exist if someone married a "parallel cousin" (who would generally be in the same clan). It may also be he case that sexual rivalries, disputes, and jealousies within a family (or clan) are disruptive and societies that allow them have not lasted.