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Why did traditional Chinese society prohibit marriage between people with the same surname?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

In traditional Chinese society, the prohibition against marriage between individuals sharing the same surname was rooted in the practice of exogamy, which mandated marriage outside of one's own family or clan.12

Because traditional Chinese society was organized around patrilineal descent, individuals with the same surname were generally considered members of the same kin group, sharing a common ancestor. Marriage was viewed as a strategic alliance intended to link different families together; therefore, unions within the same surname group were regarded as a form of incest and were strictly forbidden to maintain the social and economic stability of the family unit. While some theories suggest such taboos may have emerged from an awareness of the risks of inbreeding, the Chinese prohibition specifically functioned to reinforce the structure of the family as an exogamous unit, distinct from other social groups.3241