The Incest taboo is a practice arising out of familiarity whereby even people who have been brought up together treat each other like siblings.
Definition.com
Definition.com
The Incest taboo is a social form which accelerates cooperation among family members and reduces suspicious and hostility among family members.
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The Incest taboo is a human-defining trait. Not a biological prohibition but a social one, one which promotes dealing with and integrating with outsiders groups.
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The Incest taboo is a means of guarding against the rivalries, enmity, jealousy, and disturbance that incest would cause within the family.
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The Incest taboo is a not at all ubiquitous practice, even if rather common. It is undertaken because the sexual drive is fundamentally asocial and must be constrained for society to exist at all.
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