Inheritance rules and cousin marriage OR It is always economics

From Understanding Western Exceptionalism by Richard Hanania.  An interview between Hanania and  Joseph Henrich.

I have long been familiar with the very high level of cousin marriage in some countries of the Middle East and elsewhere.  I first came across it as an observation among developmental economists but it has obvious other social and biological implications.  While I had seen discussions about the origin of the cultural norm I don't think I have seen as clear cut an explanation as here.

But one of the interesting things about the cases you brought up, at least Egypt and Iraq, is that you get the spread of Islam through these places. And in the same way that I argue that one brand of Christianity adopted this marriage and family program, which broke everyone down into monogamous nuclear families, Islam, they constrained polygynous marriage, but then they adopted this inheritance rule. And the inheritance rule says that daughters get half of what sons inherit. Well that works fine if you’re a trader like Muhammad. But if your wealth is mostly in land, that means every time you marry a daughter off, part of your land goes away, and you get poorer and poorer and poorer as you marry your daughters off.

What Islamic society started doing was something that’s almost unheard of cross-culturally, except in the Islamic world, is patrilateral parallel cousin marriage. So if you’re a male, you marry your daughter to your brother, to your brother’s sons. And that keeps the wealth within the family and it stops that bleeding off of wealth. Those families get richer and more prosperous, so other groups copy them. But that creates this very endogenous kinship system, which is going to have the opposite psychological effect, than forcing people to build long distance ties. There’s lots of stuff going on in history, you got to take into account the role of it.