"The royal bloodline isn't what it used to be; too much inter-marrying, I suppose. I always say when you reduce a family tree to a family bush, you just can't hide as much beneath it!"Often, those of noble or royal birth are supposed to marry other nobles, typically in Arranged Marriages intended to secure alliances, but since there are so few of these people this frequently leads to marriages between cousins or similar relatives. Some even marry their siblings. Oftentimes this is used to explain The Caligula, Royally Screwed Up, and It Runs in the Family. Might be used to justify a Single Line of Descent. This is Truth in Television, even with siblings (Ancient Egypt for one).
— Regent Virini, Babylon 5.
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- In the X-Wing Series Rightfully Returning Princess Plourr Illo is surprised to see her second cousin Rial again, and dismayed by his belief that they'll go along with the old Arranged Marriage set up when they were children. Her bloodline has in-bred and been tampered with to keep it "pure", which sometimes has disastrous results.
- Preacher: It turns out the Holy Grail is indeed the receptacle of Jesus' blood... by which we mean they're the organisation who've been making the original descendants of Jesus interbreed for nearly 2,000 years to keep his bloodline pure. The results are universally retarded, deformed (the latest was born with eyes half the size of his head), anything but divine... yet somehow less physically freaky than the family of one-eyed hillbillies Jessie met and befriended.
- A Song of Ice and Fire:
- The Targaryen dynasty wed brother to sister (not as often as you'd think, but likely more often than you want to imagine), first cousin to first cousin (most common) or even uncle to niece (occasional) for centuries, which is often blamed in-universe for their tendency to produce the odd mad king (mainly by those very strongly of the Faith and rather biased on the whole issue). While the inbreeding depression was not as severe as some real-life examples, many Targaryen kings had to contend with higher-than-average rates of infertility and stillbirths, and plumbing the depths of Targaryen history will reveal more than a fair amount of royal children born with extra toes, webbed fingers, and scales.
- Cersei and Jaime Lannister are twins who secretly have an affair and three kids, with the eldest starting a three-way civil war with two secessions, though their parents were first cousins and no one thought that unusual. The good news: the younger brother and daughter are seemingly okay! The bad news: The older brother is Joffrey Baratheon. Jaime and Cersei tried to justify their relationship by citing the Targaryen practice, but both felt great pangs of guilt wondering if their sin led to their thoroughly rotten eldest.
- A few examples in the Dune universe. Though mostly intended as eugenics.
- The Bene Gesserit intended for Jessica to bear a daughter that would be mated to her cousin Feyd-Rautha as part of their Super Breeding Program, but she bore a son instead.
- Leto II symbolically married his sister Ghanima, though at that point he was physically unable to have sex so he had Prince Farad'n Corrino (who would be a very distant cousin) sire her children.
- Over the 3,500 years Leto II controlled the Bene Gesserit breeding program he mated several of Duncan Idaho's clones with his sister's descendants.
- In the second trilogy of Kushiels Legacy Imriel de la Courcel falls in love with and eventually marries Princess Sidonie de la Courcel. Imriel is the son of Sidonie's great-uncle, making them first cousins once removed.
- Chronicles of the Kencyrath: The Knorth, the royal house of the Kencyr, has a long tradition of twin marriages.
- Babar, King of the Elephants in Jean de Bruhhoff's children's books is Happily Married to his cousin Celeste.
- Harry Potter: The wizarding world doesn't have royalty, but it does have pureblood lines. As such, many of them use intermarrying to keep the bloodline pure, and disowning anyone who marries "mudbloods". Dumbledore mentions this was the case with the Gaunt family, the last descendants of Salazar Slytherin, who had inherited their family's violent temper due to interbreeding.
- In Pyramids, the High Priest Dios suggests that newly-crowned pharaoh Teppic marry any available female relative. Of course, Teppic's kingdom is the Discworld's version of Ancient Egypt.
- Parodied in Feet of Clay, where it's mentioned that in the city of Genua, the royal lines died out "through interbreeding so intensively that the last king kept trying to breed with himself"
- In The Painter Knight, several pages are devoted to explaining how an exiled member of the royal family, over the course of about 75 years, managed to tie his bloodline back into the ruling line — by marrying his cousin, having their children marry second cousins, and having their grandchildren marry third cousins — one of whom is the current monarch. His child inherits the powers of four different septs of the dynasty in one go.
- The Ciaphas Cain series partially blames the overall incompetence of much of the Imperial aristocracy (including the majority of planetary governors we meet) on an excess of inbreeding.
- In Larry Niven's Svetz stories, the world is ruled by a hereditary Secretary-General. Centuries of inbreeding have produced a feeble-minded and childish occupant of that office.
- The Masters from The Stone Dance of the Chameleon place such a high value on blood purity that brother/sister and mother/son marriages are extremely common in the imperial family.
- In Everworld, the Pharaohs of Everworld-Egypt are so inbred that the last twelve rulers have been mentally retarded. That, combined with the gods' total apathy to anything but ritual, has led to the entire country stagnating until the dwarves and the Amazons were able to take over.
- A non-royal example in Andrey Livadny's The History of the Galaxy series. The St. Ivo family runs Galactic Cybersystems, the largest Mega Corp. in history. The founder Erlk St. Ivo marries his cousin Liza to maintain the secrecy of the corporate HQ and R&D center. Deciding to continue the tradition, they had a son and a daughter, deliberately modifying their DNA so that they weren't genetically related. Each subsequent generation continued this, until the birth of André St. Ivo, whose intelligence level was far below normal (despite the genetic modifications). Deciding to break tradition and infuse the family with "fresh" blood, André's parents did not have a second child. He ended up marrying a smart young woman named Theia Mitchell. Due to her husband's inability to run a corporation, Theia ended up doing his job. Resentful, he had an assassin shoot her and then got a bunch of Mad Scientists to turn her into his sex slave, keeping her in stasis until he "used" her the next time. Due to all this inbreeding, André and Theia's sons Aramant and John ended up looking like André and Erlik, respectively. In the first case, this proved Aramant's undoing. After he found his mother in stasis and thawed her out, she assumed he was his father and killed him. John was mortally wounded and had his consciousness copied onto an android, all of whom look like Erlik (and, therefore, John).
- Not strictly royal, but the marriage practices of the Uplands are like this in Annals Of The Western Shore. The brantors (local chieftains, basically) often arrange marriages between cousins to keep from diluting the family's magic power and ensure a strongly gifted heir.
- Averted in A Brother's Price, one of the princesses says she wouldn't mind marrying her cousin, if she absolutely must marry someone, but her sister points out that he is too close in blood.
- Discussed in the third book of The War Gods series, where one man mentions that some of the people in service to Baron Tellian (Whose domain is large enough to be considered a duchy were he any nationality but Sothoii, which has no levels of nobility between 'Baron' and 'Royal') wish that he would marry his only child (Who, being female, cannot inherit directly, instead passing her claim to her father's lands to her eventual husband) to his nephew Trinial. The man he's talking to says that this would never be permitted as they were first cousins. The first man agrees, but points out that people wishing that it could happen said a lot about Trinial's character (In the end, Leanna renounces her claim on her father's lands to get away from the politics, and Tellian makes Trinial his heir directly).
- In just the third episode of The Tudors Henry VII arranged a marriage between his daughter Mary and her mother's nephew the Holy Roman Emperor Charles V, though the engagement was called off later. Note that Charles and his aunt were Hapsburgs (see Real Life).
- After all sorts of trouble was had in the Centuari court on Babylon 5, one of the ministers blames it on too much inbreeding.
- The last of the Hapsburg dynasty on 30 Rock is a prince who suffers very badly from this.
- In Warhammer it's said that during the most decadent days of the Empire the nobles were so inbred that mutations became commonplace. The Witch Hunters seem to have solved that problem though.
- The Navigator Houses in Warhammer 40,000, aka the Navis Nobilite, tend to act like aristocrats and have become so inbred over the millennia that most if not all of them have mutations other than their genetically engineered third eye (which is recessive, hence the inbreeding).
- In Ironclaw grey foxes, the ruling house of Calebria, often exhibit signs of inbreeding like hemophilia or color blindness, with rumors of more extreme traits like polydactyly.
- The novel Scars elaborates further, all grey foxes in Calebria are of House Rinaldi, and if they mate with the more common red foxes their offspring are usually red. The other major houses are the same species as most of their peasants so they have the option of legitimizing their bastards and adding some diversity to their gene pools.
- The Silver Fangs of Werewolf: The Apocalypse suffered from inbreeding despite their biological requirement to outbreed. Because they refused to mate with humans who weren't royalty, many were Hapsburgs.
- In A Man for All Seasons, the king tells Thomas More that he considers having married his dead brother's wife to be incest, despite them not being relatives by blood.
- This trope drives the main plot and two subplots in Gilbert and Sullivan's H.M.S. Pinafore. Sir Joseph Porter, the First Lord of the Admiralty, boards the Pinafore to court Josephine, daughter of the ship's captain. However, seaport floozy Buttercup declares that, in her youth, she breastfed both Captain Corcoran and seaman Rackstraw, and inadvertently mixed up the two infants. Sir Joseph then declares that he cannot marry Josephine, as she is the daughter of a mere seaman, her charm and grace notwithstanding. This allows now-Captain Rackstraw to propose to Josephine, and demoted-to-seaman Corcoran to pursue Buttercup. Sir Joseph resigns himself to courting his cousin Hebe.
- Assassin's Creed: Brotherhood: Implied Brother-Sister Incest between Cesare and Lucrezia Borgia.
- The Nanayas in Tsukihime often encourage this to preserve their innate "anything-killer" ability.
- Crusader Kings 2:
King: But the earl is my uncle, that would make her my... half-sister?
- Politely referred to as "eugenics" in the fandom, as some of the best character traits in the game are hereditary, and become more likely the more common they are in a newborn character's ancestry. The game's mechanics forbid marriage to a sibling, child or parent, but allow grandparent-to-grandchild marriages, among other icky things. All of this is thrown off the window with Zoroastrians, for whom incestuous marriages are holy, and increase their popularity with vassals. To help keep Zoroastrian families from descending into deformed messes, the game lets their rulers keep three concubines for non-inbred — but legitimate — children. The general 'divine blood' modifier that handles making incestuous marriages holy also behind the scenes significantly decreases the risk of getting the (very negative) inbred trait from being inbred... while increasing the risk of getting the lunatic trait from being inbred.
- Mentioned in the "Lust" live-action trailer.
Adviser: It's the other one sir, she's your cousin.
- Many Ancient Egyptian Pharaohs often married their cousins or in some cases sisters. King Tut suffered from any number of possible genetic disorders from being the offspring of a brother and sister, though his death has been most conclusively attributed to a malarial infection rather than any genetic cause.
- Contrary-wise the 12th Dynasty, one of the most successful in Egypt's long history married brother to sister for seven or eight generations without noticeable ill effect. The dynasty eventually failed not because of genetic damage but a lack of male heirs.
- When the Macedonian Ptolemies took control of Egypt after the split up of the Alexandrian Empire, they revived the Egyptian practice of wedding brothers to sisters to enamor themselves to the Egyptian people, and consequently to keep the Ptolemaic dynasty relatively free of Egyptian riff-raff. They also revived the Egyptian practice of brother-sister spouses ruling jointly, meaning neither party officially had more power than the other. This led to the succession battle that eventually placed the famous Cleopatra (really Cleopatra VII) as the sole ruler of Egypt despite being a woman.
- The House of Habsburg, which ruled most of Europe for several hundred years (with branches in Spain, France, England, the entire Holy Roman Empire...), was notorious for this: they acquired a lot of land through dynastic marriages and tried to avoid having the same thing happen to them by keeping things in house. The last of the Spanish Habsburgs, the infamous Charles II, was actually more genetically inbred than the product of Brother-Sister Incest would have been thanks to generations of cousin/cousin and uncle/niece marriages. His health problems were so severe that even the royal portrait artists — who were being paid to make him look good — couldn't hide his constantly protruding tongue.
- Turn of the Century British royalty:
- Queen Victoria's children were married into so many other European royal houses that by 1914, George V of England and Tsar Nicholas of Russia (first cousins) looked so physically alike they might have been twin brothers. Kaiser Wilhelm of Germany might have made a triplet.
- Queen Victoria was also a carrier for hemophilia, and the result was that most of the European royal families were hemophiliacs and carriers into the 20th century. Tsar Nicholas II's son Alexei inherited the gene for hemophilia that Victoria carried because his mother was one of her grandchildren.
- When the post-Communist Russian government wanted to verify that the human remains found in Siberia were those of the late Tsar Nicholas, they needed a close relative to match the DNA. Prince Philip, Duke of Edinburgh was tactfully approached to give DNA samples. Which proved the Romanoff DNA beyond all possible doubt.
- Queen Elizabeth II and her husband Prince Philip are 2nd cousins once removed through the Danish lines (descended from King Christian IX) and 3rd cousins through the British lines (descended from Queen Victoria).
- In ancient and feudal Japan, it was common for noble and imperial families to arrange marriages between cousins and even aunts/uncles and nephews/nieces because The Patriarch of these families tended to have boatloads of children from many different women, and this was a convenient way of tidying up familial loose ends and folding branches of the family back into the main House. In particular, it was rather common for the reigning emperor to be married to a close relative of his ruling shogun. Since the latter tended to also form dynasties, this meant that the two family trees often intertwined.
- In traditional Arab society, it is customary for cousins to marry—although often more as a last resort for a woman who has been unmarried for too long than as a first choice (such women get told, "There's always your cousin"). However, among aristocratic families, the desire to keep family fortunes together—under Islamic inheritance law, daughters are entitled to inherit (only half of what their brothers inherit, but still), and heirs cannot be disinherited except in extreme circumstances, so cousin marriage keeps the family fortune in the family.
- Eighth century Irish Princess and Roman Catholic saint Dymphna's mother passed away, and her father decided to marry her. She fled with a few companions, but when the charitable work in Belgium she was doing caught her father's attention, she and her friends ultimately were martyred, and the city whose mental health facility she started is one of the most envied, studied, and effective to this day.