In the real world, the issue with inbreeding is that it increases the risk of recessive genetic disorders. In fiction, it seems that the single most common recessive genetic disorder is evil.
This allows for the usage of tropes such as Mental Handicap, Moral Deficiency and Evil Cripple while obfuscating the ableism of said tropes. Apparently it's fine to vilify people's genetic disorders as long as their disorder was caused by incest rather than random chance.
The reasons for this trope are varied. Taboos against incest are near-universal, strong, and have only gotten stronger with time. Just as illegitimate children and children of rape have historically been shamed for their irregular parentage, so too are children of incest. Some examples overlap with those tropes, although not all—if all examples of inbreeding did fall under those taboos, there'd be no need to have a separate taboo against inbreeding.
Alternately, inbreeding may provide a convenient justification for the employment of certain tropes. Need to explain how the isolated Cannibal Clan keeps recruiting, and why its members are deformed, disabled, or mentally handicapped? Want a reason for why The Caligula is so Royally Screwed Up? Blame a recessive genetic disorder or hereditary mental illness and claim that the issues were exaggerated by generations of inbreeding!
This trope can be found on both ends of the socioeconomic spectrum, overlapping with Aristocrats Are Evil and Royal Inbreeding as often as it does with Hillbilly Horrors and rural Bandit Clans and Cannibal Clans. Very rare among the middle-class.
Sub-Trope of Villainous Lineage and Hollywood Genetics. See Red Right Hand, Evil Cripple, Mental Handicap, Moral Deficiency, Insane Equals Violent, and The Caligula for what the trope might be used to explain away. May overlap with Villainous Incest on the part of the parents.
Needless to say, this is not how inbreeding works.
- One memorable villain from The Authority was Seth Angus Billy Cletus Bubba Jamie Clement Cowie, a hillbilly given Combo Platter Powers by world governments specifically to take down the Authority (and nearly succeeded, receiving a harem of children as a reward). His birth came about around nine months after his mother was stuck in a cabin with her seven brothers, and after his defeat was transformed into seven chickens and returned to his uncles.
- Wonder Woman:
- While mythology was not a big part of the classic stories (with the few "gods" being more tied to other planets than mythology) Mars/Ares is the child of siblings and has always been a villain. Starting in Wonder Woman (1987) where the mythological characters became more important and tied to their mythological roots not only is Ares a villain all of the other Olympians (besides Hestia) have a rather wide cruel streak and they're all results of incest on some level. Zeus and Hera are quite villainous and their parents were brother and sister.
- Wonder Woman (2006): The New 52 version of Diana is the most violent and least focused on saving lives version of Diana in the main continuity, casually and gleefully killing criminals she could have easily subdued. This version also changes Hippolyta from Diana's Truly Single Parent, instead having had Diana in an affair with Zeus, who is Hippolyta's paternal grandfather, though this is not addressed in the book since incest is quite common in their family (Hippolyta is Ares' daughter).
- In the Arthurian-inspired Star Wars fanfic i've come to burn your kingdom down, Kylo Ren—antagonist of the sequel trilogy—is the son of twins Leia and Luke.
- Comes up quite frequently in Harry Potter fanfiction:
- Two Households: This is how the aristocratic Pureblood families have been reproducing for centuries, creating an increasingly shallow gene pool for themselves out of their racism and classism, with many of the bloodlines going extinct due to their inability to produce healthy offspring. This is also the group that most of the Death Eaters originate from. Even the ones who aren't Death Eaters tend to be huge assholes.
- Wish Carefully: The Light Supporters withdraw from England after Dumbledore's death, leaving the Death Eaters with what they always wanted: a magical society consisting only of Purebloods. Which will fail within another few generations as the already extensive inbreeding becomes even worse and begin producing very few children with magic.
- While not exactly evil, Pokémon Reset Bloodlines states that wild Pokémon can grow increasingly savage and violent due to generations of inbreeding, such as the Spearow and Fearow flock in the Kanto Route 1.
- The distant past of The Awakening of a Magus has Demetrius, the child of two twin Klaatu-shah, who had a serpentine appearance from birth. He manipulated his parents into a Mutual Kill at the age of sixteen, and still survives now, as the controlling mind and progenitor of the Dementors.
- The 1997 film Bleeders, loosely based on H.P. Lovecraft's The Lurking Fear, is about a group of grotesque inbred creatures who descended from an aristocratic family who isolated themselves from the world and started feeding on human blood.
- In Dark Heritage, more than a century of inbreeding has devolved the Dansen clan into a tribe of animalistic, ghoul-like creatures.
- The 1973 movie Death Line (US title: Raw Meat) features a cannibal clan hiding in the London subway system, the result of a 19th-century dig accident trapping a bunch of male and female workers in the tunnel system. By the time of the events in the film, there are only two exceedingly sickly, barely-human descendants remaining. In a twist, though, the one most centrally featured in the film is portrayed as more to be pitied than reviled, though still very dangerous.
- The villains in Evil Breed: The Legend of Samhain are an incestuous Cannibal Clan descended from Sawney Beane who are so inbred that they barely even look human anymore.
- Excalibur makes Mordred the son of Arthur and his half-sister Morgana (rather than his half-sister Morgause) and even more insane than he usually is. In the end, he not only destroys Camelot but murders his mother in a fit of rage before going down to a Mutual Kill with his father.
Mordred: Come father, let us embrace at last!
- Hot Fuzz has Lurch, a mentally deficient thug who serves as the villain's muscle and whose mother was also his sister.
- The Island (1980) is about a colony of bloodthirsty pirates largely cut-off from the outside world for 300 years. Suffering from inbreeding, most of the women have become sterile and the pirates are starting to abduct outsiders to refresh their bloodline.
- While this was absent from the original Texas Chainsaw Massacre and its sequel, later installments in the franchise declared that Leatherface's oldest brother was also his father and that most of cannibalistic Sawyer clan were products of Villainous Incest.
- Wrong Turn heavily implies this with The Mountain Men, a clan of inbred cannibal hillbillies lurking in the hills. The first film touches only lightly on the inbreeding itself, showing them as grotesquely deformed, and showing articles about inbreeding briefly in the Photo Montage over the opening credits. The sequels get into this more centrally, exploring the exact family dynamics the killers have.
- Dean Koontz:
- Whispers: "Bruno Frye" is the collective name for a pair of murderous twins whose father was also their grandfather. Left completely unbalanced by a lifetime of being sexually abused by her father and his sudden death during the pregnancy, Katherine Frye raised her twin boys to think of themselves as one person, and to see sexuality as a product of the Devil. The two become Serial Killers who hunt down and murder women they believe to be reincarnations of their now-deceased mother since they believe their genitals to be abnormal, so think they can only have sex with each other or with the one woman who knows their secret — Mom.
- The Bad Place: Candy Pollard's grandfather was an incestuous rapist who impregnated his younger sister (an event that apparently did considerable harm to her mental health). His mother was the hermaphroditic child of that union, who went on to repeatedly impregnate herself, producing four children, all with Psychic Powers. While youngest son Frank is emotionally damaged but moral, and twins Violent and Verbina are feral and amoral but not necessarily malicious, Candy is a genetically damaged, physically deformed monster, born with two sets of testicles and no external genitalia, who uses his teleporting and telekinetic abilities to become a serial murderer of the women he desperately wants to, but physically cannot, have sex with.
- What The Night Knows: The villain is the product of three generations of line-breeding, starting from a non-consensual brother-sister pairing. The father/uncle then impregnated his daughter/niece, who bore twin girls. The villain is the offspring of one twin, sired by the family patriarch.
- H. P. Lovecraft did this a few times.
- In "The Lurking Fear", the monsters terrorizing a small town turn out to be the inbred descendants of a once-prominent family who retreated underground and began marrying their own once their reputation got too ugly.
- In "The Dunwich Horror", it's heavily implied that when Yog-Sothoth fathered Wilbur and his brother on Lavinia Whateley, he did so while possessing her father Old Wizard Whateley.
- Dollanganger Series plays with this trope. Corrine eloped with her half-uncle Christophernote and they had four children. They were very nervous before their first child was born, but after having four kids and none of them having any physical or mental disabilities, Corrine is convinced that this lack of Laser-Guided Karma means that the Powers That Be have no issue with it. Corrine's mother Olivia is convinced that the kids are inherently wicked and sinful, and abuses them in a self-fulfilling prophecy to prove she was right. As a series, the books definitely come down on the side of incest kids are not inherently wicked... although they might indeed be more inclined to be incestuous themselves.
Corrine: Your grandfather predicted our children would be born with horns, humped backs, forked tails, hooves for feet—he was like a crazy man, trying to curse us, and make our children deformed, because he wanted us cursed! Did any of his dire predictions come true? No! [...] Now we had two boys, and two girls, and had tempted fate four times—and had won! Four perfect children. So if God had wanted to punish us, he had four chances to give us deformed or mentally retarded children. Instead, he gave us the very best. So never let your grandmother or anyone else convince you that you are less than competent, less than worthy, or less than wholly pleasing in God's eyes. If there was a sin committed, it was the sin of your parents, not yours.
- In Mercedes Lackey's Elemental Masters book Blood Red, the werewolf sorcerer Bertalan Kaczor is found living with his enormous, savage family, which he refers to as his "sons and wives". He doesn't refer to his "sons and daughters" because said daughters (and granddaughters!) are his wives. With the exception of Kaczor himself, the whole family is visibly deformed even by werewolf standards, which the main characters explain as the normal effects of inbreeding exacerbated by the magic they use to transform.
- In Gregory Frost's short story "Ellende", the titular Town with a Dark Secret appears abandoned at first. When the narrator Jessie finally sees the townsfolk (in the midst of a ritual for a Religion of Evil), she guesses from their appearance that the whole town has been a closed gene pool for decades if not centuries. The ending implies that the town has been in a Pocket Dimension since at least the 1920s.
- In Harry Potter and the Half-Blood Prince, Dumbledore partially attributes the mania of the Gaunt family to cousin marriage.
Dumbledore: Marvolo, his son, Morfin, and his daughter, Merope, were the last of the Gaunts, a very ancient Wizarding family noted for a vein of instability and violence that flourished through the generations due to their habit of marrying their own cousins.
- Murder in Advent by David Williams has the Daras family, who were aristocrats a few centuries back, and are now reduced to an evil old recluse and his harem of female relatives, holed up in what was their estate's home farm.
- The Jack Ketchum novels Off Season and Offspring have an inbred cannibal family stalking and killing people that they think are trespassing on "their territory".
- Olalla plays with this. When the soldier protagonist is sent to recuperate with a decayed noble family in Spain, he discovers that the family's isolation has resulted in extensive inbreeding, leaving them all with the same set of recessive traits, most notably flaming red hair, in addition to leaving the son, Felipe, and the unnamed mother, intellectually disabled. The protagonist nevertheless falls in love with the family's beautiful daughter, Olalla, and is pursuing a romance with her when he receives a cut on his hand that prompts the mother to attack him and try to eat him. Olalla sends him away after that, informing him that her family is too damaged for him to become a part of.
- Red Dragon: Graham suggests to sleazy tabloid reporter Freddy Lounds that the Tooth Fairy is the product of incest, as part of various other insults. This majorly pisses off the Tooth Fairy, who subsequently murders Lounds.
- A Song of Ice and Fire:
- Jaime and Cersei Lannister have a long-term twincestuous relationship going on, and they have three kids together. (Also, their parents were cousins.) The eldest, Joffrey, is a blatant case of The Caligula. Alternatively, this could be viewed as a Subverted Trope, as Cersei's other two children, Tommen and Myrcella, are total sweethearts with nothing wrong with them, thus implying it's not a pattern: Joffrey's just a Jerkass. (It's also worth mentioning that the actual genetic disorder that we know is present in their family — dwarfism — was not inherited by any of the three kids.)
- In-universe, the Targaryen habit of marrying their sisters is often viewed as having exacerbated the hereditary madness that crops up again and again in the family, producing a series of warped aristocrats including Maegor I the Cruel, Aegon IV the Unworthy, Aerion Brightflame, Aerys the Mad, and Viserys the Beggar King within the main family line, and equally, if not even more depraved characters like Daemon Blackfyre and Maelys the Monstrous in the bastard Blackfyre branch of the family. However there were decent Targaryens, such as Jaehaerys I, who married his sister and none of their children were outright evil, or Daeron II, the son of the cruel Aegon IV and their sister-wife.
- Phil Rickman's The Wine of Angels features a reclusive farming family whose sons are expected to lose their virginity to their mother before going out to rape and murder innocent victims.
- The Borgias: As Juan undergoes Sanity Slippage while he's dying from an STD and dulling the pain with opium, he muses about the possibility of his siblings Cesare and Lucrezia being lovers and starts to suspect that Lucrezia's child is really Cesare's and might well be The Antichrist. This puts his later endangerment of the child in front of his mother (by dangling it over a balcony) in a different light.
Juan: The issue of such a union would produce a demon to devour the worlds... and me.
- Criminal Minds: The Killer Woodsman from Season 9's "Blood Relations" is the result of an incestuous relationship between siblings Malachi and Magdalene Lee, who abandon him due to his deformities. Years later he begins stalking his parents and their new families, killing off his half-siblings in a rage at having been considered worthless.
- Game of Thrones:
- As in the novels, Jaime and Cersei Lannister are in an incestuous relationship, and as in the novels their oldest son Joffrey is The Caligula, this time with a side of sexual sadism atop his random acts of cruelty, murder, and domestic violence. Joffrey's siblings, Tommen and Myrella, however, are portrayed as good kids (again, as in the novels).
- Also as in the novels, the Targaryen family has a proud history of marrying cousin to cousin, uncle to niece, and brother to sister, producing the likes of Daemon, Aerys the Mad and his equally unhinged son Viserys the Beggar King and, ultimately, Daenerys. Perhaps notably, the two least inbred Targaryens in the show (Aemon and Jon Snow) are also the most sane.
- It's Always Sunny in Philadelphia has the notorious McPoyle clan. Between the prominent McPoyle brothers stating that they take showers together, as well as making out with their sister in public while calling her breasts "top-notch", they also have tried to frame their former gym teacher to be a pedophile simply because he was harsh on them, tricked the gang into thinking they were being held hostage, and stabbed Charlie with a knife.
- Law & Order: Special Victims Unit has Darius Parker, whose father was also his grandfather. Emotionally abused by his mother, who was never able to forget how he was conceived, Darius grew up to be a murderer and a rapist himself.
- Luke Cage (2016): Tilda starts the second season off a reasonably good person, but after getting dragged into her family's crime business and put through tragedy after tragedy, the last straw that pushes her down a dark road is learning she's the product of incest between her mother and her great-granduncle. While the series was cancelled before any plans with her could come to fruition, her actions, demeanor, and appearance in the final two episodes make it clear she was gearing up to become a major villain going forward like her comic book counterpart, Deadly Nightshade.
- Theodore "T-Bag" Bagwell from Prison Break and Breakout Kings is a Serial Killer and Serial Rapist who is revealed to be the product of incestuous rape between his father and his father's mentally handicapped sister. It's made pretty clear that T-Bag never really had a chance of being anything other than a villain, with Lloyd noting that "some machines just come off the assembly line broken." T-Bag himself revealed at one point that he was sterile, so the family line would end with him.
- In Supernatural, there have been two different episodes in which the villain ends up being an inbred human.
- In the episode "The Benders", a clan of hill folk are kidnapping people and hunting them for sport. It is heavily implied that the younger members are products of incest and likely also engaging in incest with the only female member of the clan.
- In the episode "Family Remains", a suspected series of ghost murders turns out to have been committed by feral twins born out of the incestuous rape of their mother/sister by their father/grandfather, which led to the mother/sister killing herself out of shame and the twins killing the father/grandfather out of rage. They then hid in the walls of the home to avoid detection but emerged to kill anyone who tried to inhabit the house.
- The Twilight Zone (1985): In "The Beacon", the inhabitants of the small town of Mellweather are all descended from a prominent citizen named Seth Janes who lived in the 18th Century. They believe that his spirit guides the Beacon, a lighthouse that seemingly chooses a Human Sacrifice every year, and that they must keep their bloodlines strong in order to serve him. After Dr. Dennis Barrows saves the intended victim, a little girl named Katie, he is killed by the townspeople in order to placate Seth.
- The X-Files featured an episode entitled "Home", to great controversy, about a murderous, inbred clan known as the Peacock family. The matriarch, Mrs. Peacock, is involved in incestuous relationships with her sons, one of them being the father of the other two.
- Played with in Beti-Pahuin Mythology. Akoma Mba was the son of Bela Mindzi with her brother Nlo Dzop. He was an absolutely terrifying megalomaniac and merciless Evil Overlord for most of his life. Some say he was cursed from the very beginning due to being born from such sin. On the other hand, Akoma Mba was bullied and tormented by the Esangom tribe for most of his childhood due to being inbred, it isn't too shocking he snapped under the abuse. He went from childhood bully to bloodthirsty tyrant in no time. However, over the years he calmed down and regretted much of his actions.
- Mordred is the Final Boss of Arthurian Legend, and in the most famous telling, he's the offspring of Arthur and his half-sister, Morgause. The exact circumstances of this vary, as does its effect on Mordred—older legends often have him evil from the outset, while many modern takes depict him as good until learning the truth about his parentage.
- Hunter: The Vigil: Inbreeding is offered as an explanation for the deformities of both the Freak and Mutant Slasher Undertakings, with the disfigurements either arising from genetic disorders or from the inbreeding reinforcing a supernatural taint in the family.
- Ogres are modeled on the Hillbilly Horrors and Cannibal Clan tropes, and as such, are not only willing to inbreed but actively prefer it, marrying as close to the family line as possible, with parent-on-child, sibling-on-sibling, and other combinations being a feature of ogre life and resulting in ever more deformed offspring. Their half-human relatives, the degenerate Ogrekin, are just as prone to inbreeding, and entire forests are haunted by families of Ogrekin with minds as twisted as their family trees.
- The Ogre penchant for inbreeding is inherited from their parent race, the Hill Giants, who while not as fond of it as the Ogres, are still willing to breed within the family now and then, producing some young in the process who look only barely like the standard Hill Giant. Marsh Giants, descended from Hill Giants who fled into the swamps and were reduced to reproducing with boggards, demons, and their own kin, are if anything, even more warped than the Ogres and Ogrekin, with bloodlines sullied by incest, interspecies rape, and demonic taints.
- Warhammer: Sigvald the Magnificent, the Champion of Slaanesh, was born of a particularly depraved chieftain who ended up bedding his own sister. Sigvald continued in his father's footsteps until he tried to overthrow him, his excesses bringing him to the attention of Slaanesh, and now merrily rapes, tortures, and burns his way through the world at the head of his army.
- In The Elder Scrolls V: Skyrim, Sibbi and Ingun Black-Briar could easily be seen as this. Family matriarch Maven refers to them as her children—and so does her oldest son Hemming.
- Fire Emblem: Genealogy of the Holy War: Julius was born of an incestuous relationship and bears the lineage of the evil dragon Loptous. In fact, because of how the Holy Blood inheritance system worked and the near extinction of the Loptous line, the incestuous relationship was required to produce someone with major Loptous blood.
- Wilhelm Ehrenburg from Dies Irae was born as a result of his father raping his sister which lead to him being born albino. Due to his albinism he ended up despising the light and started idolizing vampires, a fantasy he eventually would lead to try and fulfill starting with murdering his parents and going from there.
- Sayo from Umineko: When They Cry is the product of her grandfather raping his illegitimate daughter due to believing that she was her mother's reincarnation. As a result, Sayo was hidden away, which is one of the reasons she's so messed up. While most likely not outright evil, Sayo is the Greater-Scope Villain of the series.
- Mordred in the base arc of Arthur, King of Time and Space, in the space arc he's still inbred but not particularly evil, while in the modern arc he's Guinevere's son and a little creepy.
- SCP Foundation: SCP-3288 ("The Aristocrats") is the (literally) monstrous bloodline of Leopold I, who used eldritch magic to make his descendants highly resilient to incestual genetic defects. The magic succeeded in suppressing inbreeding-related disabilities such as infertility, illness, and retardation, but did not suppress other kinds of genetic defects such as deformities and mental illness, and now said descendants are insane, cannibalistic, narcissistic mutants.
- In Ben 10: Alien Force, the Highbreed, an alien species so obsessed with genetic purity that they seek to exterminate all other races due to their "inferior" gene pool, are stated to all be inbred and sterile. Azmuth knows that their Cultural Posturing is an ironic veneer and pities them.