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. This is Truth in Television, even with siblings (Ancient Egypt for one).
- In A Song of Ice and Fire the Targaryan dynasty wed brother to sister for centuries, often blamed in universe for their tendency to produce mad kings. Later Cersei and Jaime Lannister are twins who secretly have an affair and three kids, which ends up starting a five-way civil war, though their parents were first cousins and no one thought that unusual.
- 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 Kushiel's Legacy Imriel de la Courcel falls in love with and eventually marries Princess Sidonie de la Courcel. Imriel is the son of Sidonie's 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.
- 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 II, though the engagement was called off later. Note that Charles and his aunt were Hapsburgs (see Real Life).
- 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 Forty Thousand, 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 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.
- The Ancient Egyptian Pharaohs often married their cousins or in some cases sisters. Some suspect that King Tut died so young because of several generations of inbreeding, and his only children were stillborn because of it.
- 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.
- 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 looked so physically alike they might have been twin brothers. Kaiser Wilhelm of Germany might have made a triplet.
- 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. Members of the British Royal Family were tactfully approached to give DNA samples. Which proved the Romanoff DNA beyond all possible doubt.
- Nicholas II's son Alexei inherited the gene for hemophilia that Victoria carried because his mother was one of her grandchildren.
- In ancient and feudal Japan, it was common for noble and imperial families to arrange marriages between cousins 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.
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