When did incest become a social taboo? | AskHistorians Archive Observer

The ban on marriage with a relative is actually one of the most universal social rules in the history of mankind. Associated with this ban, incest has been a social taboo or even subject to harsh punishments in almost every human society we know off. Especially incest with a first-degree relative was rarely, if ever, acceptable. Anthropologists suspect that there's a very specific reasoning behind this. Namely how marriage -the broad definition of the word- was to be used to bind strangers and form alliances. If you would marry a relative, you and your relatives would not benefit from this arrangement. If you were to marry a stranger however, both parties heavily profit. This principle is considered to be one of the cornerstones of human society.

Considering the straightforward nature of the question, I found it rather difficult to alter or elaborate my answer without going off-topic or plagiarizing sources. Hence why I've chosen to directly quote an article by Françoise Zonabend instead. For a more elaborate answer, you'll either need to pose a more elaborate question or read the article yourself. It's a very interesting article and it elaborates on notions already posed by renowned anthropologists - such as Lévi-Strauss. To properly place these paragraphs, I'd urge anyone to first read the top comment in this thread, as already posted by /u/SearocksandtreesKeep. In these paragraphs, the author is responding to a long-lasting argument within anthropology and that comment will help you interpret the subtleties. She states that :

"While the prohibition of incest is universal, it is defined differently in each culture; it is this that reveals it's social nature. In other words, its universality makes it a natural characteristic, but the mulitple forms it takes turn it into a cultural phenomenon; in this way, the prohibition of incest effects the transition between natural state and cultural state, indicating, as it were, the link between the natural fact of consanguinity and the cultural phenomenon of marriage. In animal societies, nature may well abandon unions to chance or arbitrariness, but it is impossible for human societies not to introduce order or rules, although these rules may differ from scoeity to society and from age to age."

In the same article she continues with :

"There are other known cases, in other societies, of prescribed unions that we would consider incestuous, generally within royal lines where there was a concern to preserve the purity of the 'blood'. However that may be, none of these phenomena invalidates the universality of the prohibition of incest, which above all reflects the need for a group to avoid becoming isolated and closed in on itself by forging alliances with other groups."

Source:

Françoise Zonabend, An Anthropological Perspective on Kinship and the Family, Paris, 1996.