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Anthropologically it probably emerged when people started to notice that children of siblings have birth defects more often than other families.
Genetically, inbreeding tends to magnify recesive traits, and can lead to serious problems in the offspring.
This creates a social bias against incest, and it becomes cultural.
Once it's taboo, people stop thinking about WHY it's taboo.
Given the use of contraception, there is no MEDICAL reason why people souldn't all be motherfuckers and so on; however there are also issues of coercion and force - in a family it's hard to show that volition was truly there because you can't rule out conditioning and so on.
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Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
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