The most famous example of inbreeding is in ancient Egypt, where several Pharaonic dynasties collapsed after a couple of hundred years. In order to keep wealth and power within the family, the Pharaohs often married their own sister or half-sister and after a handful of generations the offspring were mentally and physically unfit to rule. Another historical example is the royal houses of Europe where royal families often married among each other because tradition did not allow them to marry people of non-royal class.
The high amount of mentally retarded and handicapped royalties throughout European history shows the unhealthy consequences of this practice. Luckily, the royal families have now allowed themselves to marry for love and not just for status.
The Muslim culture still practices inbreeding and has been doing so for longer than any Egyptian dynasty. This practice also predates the world’s oldest monarchy (the Danish) by 300 years.
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