Incest Standards Are Relative

"There is not some sort of trans-historical incest [definition] that has always existed."

— Brian Connolly, "Incest", Betwixt The Sheets

Incest is defined as romantic or sexual relations between people sharing a familial connection and is a cultural taboo across different societies. The most common no-no is incest between the nuclear family (i.e. parent/child or sibling/sibling), which is illegal in most countries nowadays. The legality of cases like Kissing Cousins or non-blood-related incest like Wife Husbandry, Not Blood Siblings, or Flirty Stepsiblings may vary, but those usually still carry a stigma. The exact definition of what counts as family and thus also incest can however be very different across cultures and time periods. A romantic relationship between the following may thus also be defined as incest and taboo:

• Between in-laws (e.g. by marrying your dead brother's wife or her sister; see also Double In-Law Marriage).
• Between milk siblings (people nursed by the same woman).
• Between Sworn Brothers.
• Between people blood-related in a specific way (like between an uncle or aunt and a niece or nephew), through a specific member of the family, or by being on the same side or branch of the family. Marriage between family members outside of these specific relations may be seen as fine in that culture (see also the concept of parallel versus cross-cousin marriage).
• Between people who grew up in the same creche, kibbutz, or other cultural in-group (see also the concept of the Westermarck Effect).
• Between people with a religious or spiritual connection (e.g. one being a godparent to another).
• Between step-siblings, even retroactively.

If one or more of these definitions of family is normal or widespread in the culture of the creator of a work, the Angst and drama caused by this kind of Forbidden Love may cause Values Dissonance in readers not brought up with that taboo. Culture-specific incest may also be played for Deliberate Values Dissonance, especially from an outsider's POV or in a constructed fantasy culture — then Your Normal Is Our Taboo is in play. The definition of what counts as incest may also be different for royalty or the divine compared to the common folks.

Subtrope to Forbidden Love. See also Incest Is Relative.

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Examples:

Literature

• In one of Fazil Iskander's stories, a person is being blackmailed with a secret so terrible that his son is certain to commit suicide should he learn. The secret is that long ago, before the son married, his father and his future mother-in-law used to be lovers. Both were widowed at the time, and there were no children born from the affair, but still makes their kids' marriage incest by Abkhasian standards.
• Always Coming Home: All the Kesh people belong to one of five "Houses", determined by the mother's House. Sex with a member of the same house is a strict taboo. One story describes a man who raped a woman and immediately killed himself, with the author commenting that in the original, untranslated story, the way he addressed the woman makes it obvious from the start they are from the same House.
• Ascendance of a Bookworm: In addition to having a Not Blood Siblings attitude similar to real-life Japan, Yurgenschmidt only truly considers children siblings if they share a mother. Alstede's husband is also technically her paternal half-brother and Ferdinand can get engaged to a blood-related niece because he only shares a father with her mother.
• Citizen of the Galaxy: The Free Traders divide their boats in moieties, crew of sailors treated as relatives among whom no marriage is allowed. For instance, on the Sisu, when Mata grew close with the main character Thorby (who originated from a different culture, and was not blood-related to Mata), the Chief Officer actually sent Mata to another ship, since both belonged to the same moiety.
• The Dark Tower: Mia, who possessed Susannah, has a conversation with her host, revealing that she was the succubus who had taken Roland's seed and used it to impregnate Susannah when they were confronting the Speaking Demon when they were retrieving Jake. Mia calls it an incestuous encounter. Susannah protests this, saying Roland is no blood kin to her. Mia points out that the term the ka-tet uses for Roland, dinh, not only means leader or king, but that it, as well as its definitions, all boil down to mean "Father".
• Hainish universe: On planet O, people are divided into "Moieties" known as Evening and Morning, and all of society is built around said moieties. Sexual relationships within a moiety are forbidden. A typical marriage involves two men and two women, one from each moiety. As such, each person has two people he is married to, a man and a woman, whom he is supposed to have sex with, and one of the opposite with whom he cannot have sex with. No one in the books ever even considers it.
• The Ingenue: The titular character cannot marry Miss de Saint-Yves because she accepted to be his godmother for his bapticism, causing the major part of the intrigue to obtain a Papal dispense.
• Judge Dee: In "The Chinese Maze Murders", General Ting's son turns out to be having an affair with a married woman his age. Unfortunately, said woman is his father's concubine, which is considered as bad as incest even without the (attempted) Murder the Hypotenuse (Ancient China did not like patricides, even against an Asshole Victim). The judge orders the man to commit suicide to prevent the family's corruption from spreading any further.
• The Left Hand of Darkness: Downplayed with the Human Subspecies on Gethen as a side effect of their monthly heat cycles and historically isolated communities. It's not considered incestuous for siblings to have a sexual relationship and even have one child together. However, they're required to break things off at that point — anything more is strictly taboo.
• In Mammoth Hunters by Sergey Pokrovsky, the main antagonist is a man who married a woman from the same clan. While not actual siblings, it still made them brother and sister for the society, forcing the couple to become outcasts. The only reason they manage to survive in comfort is because, through a fluke, the man got the reputation of a powerful sorcerer.
• Percy Jackson and the Olympians: The series deals with Classical Mythology, so Divine Incest is part and parcel of that. Relationships between the gods' semi-divine children are also prevalent and accepted. In the 5th book, it's handwaved when Percy explains that since the gods don’t have DNA, it's considered fine. There is one taboo, though—it’s not acceptable for campers to date someone from their own house (meaning they share a divine parent).
  Percy's narration: And I know some of you might be thinking: aren't all demigods related on the godly side, and doesn't that make dating gross? But the thing is, the godly side of your family doesn't count genetically speaking, since gods don't have DNA. A demigod would never think about dating someone who had the same godly parent. Like two kids from Athena cabin? No way. But a daughter of Aphrodite and a son of Hephaestus? They're not related. So it's no problem.
• In Poltava by Alexander Pushkin, as well as the opera based on it, Kochubey absolutely forbids marriage between his daughter Maria and Mazeppa first and foremost because Mazeppa is Maria's godfather, so it will be a form of incest. In the opera, Mazeppa points out he can get permission for the marriage from the archbishop^note Which is possible in Orthodox Christianity in Real Life for godparent/godchild marriages, but it doesn't convince Kochubey. Maria runs away with Mazeppa anyway, but their relationship ends in tragedy when Mazeppa has Kochubey killed for political reasons.
• A Song of Ice and Fire:
  • Avunculate marriage is the blurry edge of incest in Westerosi society. Sibling marriage is definitely forbidden; cousin marriage is definitely allowed. Aunt/uncle and niece/nephew? It's murky. Incestuous marriage (most commonly Brother–Sister Incest) was common among dragonriders of Old Valyria to keep the bloodlines pure. When Valyrians Aegon and his two sister-wives arrived in Westeros, their union was in defiance of the doctrines of the native Faith of the Seven, who saw the products of such close incest as abominations. Early in the Targaryen reign, the Faith of the Seven protested the marriage of Maegor and his half-niece Rhaena on the grounds of incest. But there are also examples of native Westerosi couples with the same half-uncle and niece relationship: Edric and Serena Stark, and Jonnel and Sansa Stark.
  • Unlike other cultures in Westeros, which tolerate cousin marriages, the Free Folk are forbidden from marrying other members of their own clans. Men are expected to search far away for a wife and bring her back to the clan. The Free Folk are aware of the health issues associated with inbreeding, though they attribute it to a punishment from the gods. Ygritte explains to Jon that Longspear Ryk is practically her brother because they're from the same village, therefore a romantic relationship between them would be considered incest.
• The Stormlight Archive: The Vorin religion has a taboo against remarrying to a relative of a former spouse, under the perception that in-laws are related. Though Navani and Dalinar clearly care for each other, her prior marriage to his brother Gavilar means this proves an obstacle to their romance, and even though the literal Stormfather officiates their marriage, it ultimately is a factor in Dalinar's ex-communication from Vorinism.
• In ''A Time of Changes" by Robert Silverberg, the narrator has been attracted to his "bondsister", Halum, all his life, even going as far as marrying her very similar-looking cousin. Once she learns of his feelings, Halum is so disturbed she is Driven to Suicide at once.
• War and Peace: Sonya is secretly delighted when Andrey and Natasha reunite because Nikolay can't possibly have an 'incestuous' relationship with Marya; this despite the fact that Sonya is his blood relation (first cousins), while Marya is only his brother-in-law's sister. Which, to clarify, made her his actual sister by the standards of the time, while a cousin is just extended family and therefore at least one step removed.
• The Wayfarer Redemption series: The ruling family of the Icarii race, the SunSoars bear a curse that they will always be drawn — both romantically and sexually — to others of their bloodline; Love Cannot Overcome is in full effect for relationships outside the family, which near-invariably fail as their blood, “will crave another SunSoar”. Although Kissing Cousins, aunts/nephews & uncles/niece, or even child/grandparent are acceptable (as the Long-Lived race cease to physically age out of their prime), they nonetheless have a taboo on parent-child or Sibling Incest. A subplot in the later books reveals that Caelum, eldest child and heir of Axis and Azhure, was in an illicit relationship with his sister RiverStar before she was murdered — supposedly by her own twin, Drago (formerly called DragonStar). When this resulted in her becoming pregnant she pressured him about being open about their relationship, on threat of claiming to their parents that he’d raped her; Caelum, internally struggling between this and his public image, fell into a frenzied rage and stabbed her to death and when Drago caught him in the midst of it, he modified his memories to frame him for the act.
• White Indian series: The second El-i-chi is considered by the village matrons to want an incestuous marriage with the daughter of the man his mother married after El-i-chi's father was murdered.
• The Wolfhound: A girl doesn't want the guy chosen for her as a husband. He, thankfully, isn't so keen on the idea either. So the two publicly perform a form of food sharing which — in their society — makes them officially siblings.

Live-Action TV

• Cleopatra (1999): Caesar's solution for the power struggle between Cleopatra and her brother Ptolemy is for them to marry. While this would be considered incest in Roman society, Caesar says that there is precedent in the Egyptian royal family.
• Community: In "Wedding Videography", Garrett and his bride Stacey discover — at the wedding no less — that they're actually second cousins. Everyone present, themselves included, initially treat this as a dealbreaker (despite the fact that second-cousin marriage is perfectly legal, even in states where first-cousin marriage isn't), though they do ultimately decide to go through with it anyway. The episode ends with a monologue by the (supposed) writer about the moral and ethical issues surrounding cousin marriage and the inconsistent US laws regarding it, a matter which seems to be of personal concern to him (it's all but outright stated that he's sleeping with his own cousin).
• Derry Girls: Played for Laughs in episode 2.04 "The Haunting". Michelle walks in on her cousin James and her best friend Erin kissing and calls it "incest". Culturally it totally isn't, but what she's getting at is that it feels incestuous to her — all three of them are part of the same close-knit friend group, James is her blood cousin, and Erin is like a sister to her.
  Michelle: This is incest!
  James: No, it's not.
  Michelle: Yes, it is, James.
  James: We're not related.
  Michelle: Oh, and that makes it okay, I suppose?
  James: Well, it makes it not incest.
• Gilmore Girls: Played for Laughs when Lorelai discovers her paternal grandmother's maiden name was also Gilmore. Emily confirms that Trix and her husband were second cousins. Lorelai and Rory are totally squicked out, and a little perturbed that Emily isn't.
  Emily: Oh, don't act so scandalized. It was not at all uncommon for prominent families to keep the bloodlines closed.
  Lorelai: Keeping the bloodlines closed. Is that what we're calling it?
  Emily: Well, what would you call it?
  Lorelai: Oh, I don't know. How about "Good morning, Appalachia, I got a mighty cute sister and an extra set of toes."
  Emily: No one has any extra toes.

Mythology & Religion

• The Book of Genesis:
  • Jacob married the sister of his then-alive wife. Subsequent Israelite standards, codified in the Book of Leviticus, forbid a man from marrying his wife's sister until the death of his wife. The Book of Numbers says that Amram, father of Moses, married his father's sister, which is also forbidden under the Leviticus code (which was given over 83 years after Amram's marriage).
  • At one point Abraham goes to the land of Gerar. While there, he lies, claiming that Sarah is his sister. When the lie is discovered, he claims that he didn't really lie—Sarah was both his wife and his paternal half-sister. So apparently he expected the king of Gerar to find it acceptable for paternal half-siblings to marry.
• One Muslim legend tells of a ruler who wanted to take one common man's wife for himself. Upon realizing the intention, said man served the ruler a drink... and then revealed his wife contributed a certain ingredient. The ruler said "That makes your wife my mother!" and went home.
• The story of Saint Olga states that she, when a pagan and a widow, caught the eye of Emperor Constantine VII. She insisted on being baptised with him as her godfather, giving her grounds to refuse him as his daughter.

Theatre

• Hamlet: Hamlet frequently accuses his usurpurous uncle King Claudius of engaging in adultery and incest by marrying his late brother's wife (the one he murdered). While unusual, it appears that the Church does not consider Claudius and Gertrude's marriage to be unlawful.
• The Marriage of Figaro: Cherubino's woes come from his growing attraction to the Countess. She isn't related to him, but as his godmother, there can be nothing between them. In the sequel play The Guilty Mother, it turns out they did sleep together once, with Cherubino deliberately getting himself killed in battle but leaving the Countess with Someone to Remember Him By.
• Six: The Musical: Discussed in "No Way", since Catherine of Aragon mentions one of Henry VIII's reasons for annulling their marriage was that Catherine was first married to his late brother (never mind the fact that Catherine didn't choose to marry either man). Subverted, since it's made clear Henry's just making excuses so he can marry someone else.

Video Games

• Crusader Kings II: What "counts" as incest and is therefore prohibited depends on characters' religious preference. Eastern Orthodoxy outlaws "avunculate" (aunt-nephew or uncle-niece) marriages, but Roman Catholicism doesn't, and religions with the "Divine Blood" doctrine (Zoroastrianism, Messalianism, and reformed pagan religions depending on player or AI choice in Holy Fury) actively encourage marrying sibling to sibling or parent to child.

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