I remember hearing at some point that madness became common amongst European royalty due to the lack of genetic variety in the royal gene pool and that it affected the monarchy's power. Is there any truth to this? Did it happen in multiple countries? What about outside of Europe?
The Hapsburgs used marriage as a tool to gain a lot of thrones, but after they had much of Europe, under Emperor Charles V, they found themselves marrying within a reduced gene pool, and had a real problem of consanguinity. The same "Hapsburg lip" or deformed jutting lower jaw, that made it hard for Charles to eat continued in his line. But it was the last of the Hapsburg Spanish kings, Charles II who was not very functional- he was the only one you could call insane, and even he was able to pass the throne to the son of Louis XIV ( with some French pressure) on his deathbed.
A specific case outside of Europe which I'm curious about is the Japanese Imperial family during the Heian period. I've heard that the Fujiwara family exercised control over the Emperors by repeatedly marrying their daughters into the Imperial family. Is there any record of this leading to Emperors with birth defects?
Another non-European instance which I am curious about is Ancient Egypt. I've heard people speculate that since sister-brother marriage was common it led to huge defects in the royal family (for example, the [weird, alien-like way Akhenaten's body is portrayed may be showing birth defects] (https://upload.wikimedia.org/wikipedia/commons/4/47/Akhenaten%2C_Nefertiti_and_their_children.jpg)). Is this the view of historians?
