Framing
The handling of incest in general often has a duality to it. It's either a marker of:Hyper-promiscuous vs hyper-selective
- Hyper-promiscuous: "Not related" is framed as the most baseline criteria for sexual partners. The absence of that one criteria implies the absence of all other criteria, and a willingness to have sex with anyone.
- Hyper-selective: Either a "no one else is good enough for us" sort of elitism (sometimes royal, sometimes merely arrogant), or a sort of "you're the only one for me" Single-Target Sexuality.
Either way, it's rarely paired with the potential for "normal" romantic/sexual relationships with non-kin.
Extreme poverty vs extreme wealth
- Extreme poverty: Incest associated with particularly rural poverty — Hillbilly Horrors, Bandit Clan, Cannibal Clan
- Extreme wealth: Incest is associated with royalty, as well as aristocrats and the very rich.
Either way, incest is rarely associated with the middle-class.
Theories
You can read about the commonly cited pair of phenomena: Westermarck Effect and Genetic Sexual Attraction.The Genetic Explanation
Nowadays, genetics is the main explanation cited for the taboo against incest. However, before a modern understanding of genetics emerged, the taboo was explained in terms of religious prohibition and/or social roles. Even now, the genetic explanation is patchy. Unrelated couples are not immune to having children with genetic disorders. People may rely on this explanation solely because a social and psychological explanation to our aversion to incest has not been fully fleshed out and/or available to the average person.What qualifies, scientifically, as "incest" varies culturally. When looked at cross-culturally:
- "Hard" incest: The prohibition of parent–child and sibling unions (sharing 50% of genes) is virtually universal.
- "Mid" incest: Unions between aunt/uncle–niece/nephew or half-siblings (sharing 25% of genes) are usually prohibited, but not universally.
- "Soft" incest: Cousin unions are allowed more often than they're prohibited.
- Some cultures split cousins into "parallel cousins" and "cross cousins"
, where cross cousins are marriageable and parallel cousins are not. Genetically, there is no distinction.
- Some cultures split cousins into "parallel cousins" and "cross cousins"
First cousinsnote share 12.5% of genes. For them the risk of having babies with genetic disorders is present, but minor. Second cousinsnote share 3.13% and third cousinsnote share 0.78%. Genetically, these pairings are not closely related.
The problem of inbreeding is when it is repeated throughout generations. The percentage of shared genes compounds over time. Let's say cousins Alice and Bob marry and have 2 kids, Carol and Dan. Carol and Dan are siblings (50%) but they're also second cousins (3.13%). So they share 53.13% of their genes. Repeat for several generations, and it eventually adds up to sharing the same genes again.
The most infamously inbred royal of European history, Charles II of Spain, was the result of many generations of frequent marriage between cousins and uncles/nieces. Or for another example, the last surviving population of mammoths—the Wrangel Island mammoths—were an isolated population of about 300 individuals, where they were all de facto related. Fast-forward about 6,000 years, the harmful mutations built up, until they eventually suffered a mutational meltdown and went extinct.
