Posted by /u/holomorphic_chipotle

https://www.reddit.com/r/AskHistorians/comments/1mm8ef9/what_is_the_history_behind_the_incredibly_high/n825ndb/

You asked about inbreeding and consanguineous marriages, but I think there is an even more important distinction we need to make first. As you rightly pointed out, consanguineous marriages have historically been quite common. The ocassional marriage to a second-degree cousin is not particularly dangerous — you share about as much DNA as with the average person — so while I wouldn't advocate consanguineous marriages [personally, I've never understood friends who only dated within our close circle of friends], I think the widespread disapproval of any kind of cousin marriage that you find in South Korea and the United States is somewhat exaggerated. What makes the Middle East somewhat different is the acceptance of first cousin marriages, specifically parallel cousin marriages.

So what is a parallel cousin? In anthropology, the children of a parent's same-sex siblings are called parallel cousins: both the children of your mom's sister(s) and the children of your dad's brother(s) are your parallel cousins. In contrast, the children of your dad's sister(s) and the children of your mom's brother(s) are your cross cousins. This is sometimes related to inheritance systems. For example, in some historical West African societies, only males could own land, yet inheritance went through the maternal line; this would have meant that if you maternal uncle passed away, you, and not his children, would inherit his land — assuming, of course, you were a male, for what would the past be without old-fashioned sexism?

In some cultures, you are expected to be closer to your cross cousins than to your parallel cousins, in other cultures it is the other way around, and in some others it makes no difference. Human societies have a miriad of ways to organize so I don't want to speak in absolutes, but some anthropologists have noted that in societies where both men and women inherit (or are given land as dowry), father's brother's daughter (FBD) marriages avoid the problem of family land being split. A more extreme form of this are uncle-niece marriages, of which the Habsburg became experts.

Andrey Vitalievich Korotayev is a Russian anthropologist and scholar of many different fields (politology, sociologist, economic historian) who has studied the prevalence of FBD marriages around the world. I will say upfront that I do not fully agree with his theories, which I feel are very close to cliodynamics (u/mikedash collected several posts that discuss this field); however, he did study this topic and it would be dishonest not to present his findings.

In 2000, he published Parallel-Cousin (FBD) Marriage, Islamization, and Arabization (DOI:10.2307/3774053), in which he found that by adjusting, combining, and correcting data taken from Murdock's 1967 and 1990 Ethnographic Atlases, he could rearrange the geographical groupings to show that the prevalence of FBD marriages is concentrated in the territories of the former Caliphate. Although he does not specify which one and refers to it as both the "Arab Khalifate" and the "Islamic Khalifate", and notes that 66% of the south-central Asian cultures studied also had preferred FBD marriage, he states:

There was no doubt that an area's inclusion in the eighth-century Arab Kahlifate (and remaining in the Islamic world afterwards) is one of the strongest possible preditcors of FBD marriage. But why?

Korotayev, 2000, p. 400

His explanation is that Islamic law granted inheritance rights to daughters, equivalent to half of those granted to sons [in middle school, one of my math tests had the question: How should a deceased man's estate be divided among his two children and wife if the son receives twice as much as the daughter, and the mother receives an amount equal to the average of the two children's shares? Answer: 4/9, 1/3, and 2/9]. While this norm did not cause problems in the Arab mercantile and nomadic culture, it created problems among the agricultural populations conquered by the Caliphate.

Korotayev also argued that FBD marriage was a cognitive solution that first emerged in Syro-Palestine before the common era, became a marriage pattern widespread among non-Islamic cultures (he mentions the Druze and Maronites), and spread throughout Arabia via Jewish influence. For this he cites the work of Patricia Crone, a very influential scholar thanks to her emphasis on source criticism and one of the initiators of the Revisionist School of Islamic Studies (see Hagarism: The Making of the Islamic World), whose wider conclusions have nonetheless been rejected; she herself admitted years later that it was simply a hypothesis and not conclusive [by now, quoting Crone's early work about Islam uncritically is almost a give away that the author lacks familiarity with the historiography of early Islam]. This is another reason why I am not fully persuaded by Korotayev's explanation.

In any case, Korotayev linked the spread of FBD marriages, which appear to be unrelated to Islam in origin, to the processes of religious conversion and Arabization. First, conquered groups slowly converted to Islam, and then, over time, Arab norms and cultural practices were borrowed by the non-dominant Muslim groups present in the Caliphate. Though I have my doubts, I cannot evaluate whether this timeline of dual long-term processes is feasible. I will also note that this theory fails to explain the prevalence of FBD marriages in Muslim countries where little Arabization took place: While they exist in Central Asia, they are uncommon in Indonesia.

As for the Habsburg jaw, poor Charles II of Spain is an extreme case because his paternal grandfather was the result of an uncle-niece marriage, his maternal grandfather the offspring of first degree cousins, one of his grandmothers was a niece of the other, and his parents themselves were an uncle-niece marriage. This level of consanguinity is several times more extreme than the usual FBD marriage.

Reference:

Korotayev, A. (2000). Parallel-Cousin (FBD) Marriage, Islamization, and Arabization. Ethnology, 39(4), 395–407. DOI: 10.2307/3774053

https://www.reddit.com/r/AskHistorians/comments/1mm8ef9/what_is_the_history_behind_the_incredibly_high/n825ndb/

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