> Today in much of the non-western world first cousin marriage is still normal. I believe most arab marriages are to first cousins

First and second cousins, and only in a few countries, and much of the collected data you'll find on the topic is 40-50 years old[1]. But what does that have to do with China?

> Dunno if it's really true, but it does seem to fit various historical developments

This is a really dangerous line of anthropological reasoning; certainly non-scientific at the very least.

[1] http://www.consang.net/index.php/Main_Page has a long bibliography. Unrelated: the site feels a little creepy...I don't totally understand the purpose of having a standalone site for the topic. However, the citations appear legitimate.