Royally Screwed-Up



""Madness and greatness are two sides of the same coin. Every time a new Targaryen is born, the gods toss that coin into the air and the world holds its breath to see how it will land.""

- Barristan the Bold, A Song of Ice and Fire

""It's as if the dictum 'power corrupts' heads to the gonads.""

- Troper Camacan

Nearly every family of a decent size has at least one relative who's a little... strange. Maybe it's Great-Aunt Enid and her collection of carefully mounted cat skeletons (no one knows where she gets them—they just appear), or second cousin Dolf's extensive research library on famous serial killers, complete with memorabilia he buys off of eBay at outrageous prices. (Those clown paintings he adores are particularly creepy.)

Not much of a problem, usually, as long as one is careful not to get cornered by them at family reunions - but what happens when your family are hereditary rulers of some kind? Kings, Emperors, High Priests, whatever you want to call it, the point is you have power. Power that belongs to your family, and your family only. Sometimes, due to random chance - or not-even-remotely random deliberate action - Great-Aunt Enid or second cousin Dolf ends up with the royal prerogatives.

What follows is a reign of grotesque excess, blood, and terror. Eventually, however, King Dolf or Queen Enid will leave the throne, unless they've done something extraordinary... That should be the end of the problem, right? Not necessarily. In fact, not even probably. Dangerous insanity in the ruling line rarely appears in a single isolated case. Nope. Chances are the whole family line is just as tainted somehow, which means that sooner or later - probably sooner - along will come Queen Enid II and King Dolf III, and the whole mess will start up all over again.

This may continue for a good many decades or even centuries, with each new generation crossing its fingers that they get one of the "good" rulers from the line and not one of the "iffy" bunch. If you are unfortunate enough to get stuck with one of the blood-drenched loonies, one common solution is to go find someone else from the same family who didn't get hit with the crazy stick, or at least not whacked quite so hard, and put them on the throne instead of Enid or Dolf. This is where disgraced half-brothers and exiled princes/princesses come into play. Unless something permanent is done about the family problem, however, this is most likely just a temporary solution. Give it a generation or three and it's back to Queen Enid IV and King Dolf VI.

One not-at-all common solution is to just get rid of the "absolute hereditary power in the hands of a single individual" government setup, but that's easier said than done; switching from monarchy to republic (be it aristocratic or democratic) on the fly is a tricky job best not undertaken by amateurs and perhaps not possible at all depending on your country's power structure, economic setup, or general level of civilization. (A more feasible solution would be the replacement of absolute monarchy with constitutional monarchy, or at least elective monarchy- but somehow nobody seems to think of that outside of real life. Of course, even in the real world it took millenia, plus the right set of legal, cultural, and social conditions, for these ideas to take hold.) Sometimes the problem can be dealt with by swapping out the old family and swapping in some new group, but check the new line carefully for nasty skeletons in the closet before you give them the keys to the kingdom. Generally, though, if you're serious about fixing the "recurring insane monarch" issue, you'll have to figure out what's at the root of the problem and deal with it.

In fiction there are several common reasons why a royal family might be prone to madness.


 * Genetics: It's In the Blood in the completely literal scientific sense. The issue is strictly genetic. Usually, that means excessive inbreeding, sometimes very excessive. Sometimes, the initial problem wasn't inbreeding but genetic damage by an outside source that was intensified and cemented into the royal line through inbreeding after the fact. In any case, the family just has a crazy streak built in, and you're not getting rid of it unless your society is advanced enough to have genetic engineering (or a magical equivalent thereof) to deal with the problem or egalitarian enough for the high nobility to stop marrying each other. That last bit ought to help, eventually.
 * Moral Lamarckism is the classic magical version. The moral failings of your forebears express themselves in a taint on your own soul, like a kind of spiritual gene. Functionally, there isn't much difference.
 * Family Curse: Worse than crappy genes. Someone has cursed the royal line somehow. This can easily be a lot nastier to deal with than a simple problem of bad genes, because even if you're careful about the inbreeding, the curse doesn't care. It may even spread out to people who marry into the royal line and cause them to go mad even though they're only family by marriage, not by blood. It also means that you might not solve the problem by just picking a new family to rule over you - they're likely to get swatted by the curse just as soon as they take power. Obviously, to fix this you need to figure out who or what cursed the royal family and why, and deal with it by whatever means necessary. You could try jumping straight to a parliamentary system and see how the curse deals with having hundreds of "rulers" - but you'd better hope it just doesn't spread out to cover them all or it will make your old problem seem laughably trivial by comparison.
 * One variant of this is a spiritual imbalance brought on by upsetting the planetary equilibrium. This works just like a curse, but is the result of natural processes rather than deliberate magic.
 * Cultural: The madness is the product of nurture, not nature, which means exiled princes will be fine, at least for the first generation. If they don't change the culture that produced the madness, it will return. Possible reasons include:
 * The family has just gotten too used to being pampered and in power, and each generation has gotten a little more corrupt and decadent until finally people started to notice.
 * The culture expects its rulers to be "divinely touched" and requires the king to be at least a little crazy.
 * The culture itself is so hard on its rulers that not being paranoid and vicious means your reign will be measured in months, if you're lucky. In which case, you only look insane to cultures outside your own. Within your own realm, madness is just a survival strategy.
 * The very way the royal kids are raised is detrimental to their sanity.
 * Environmental - Some X-factor specific to the royal family's home location, diet, or environment.
 * Heavy metal poisoning, especially lead. Seriously; it's a fashion at the moment for forensic archaeologists to imply this as the cause of most of the real world cases of mad monarchs 1500-1815.
 * Disease. Specifically, syphilis; it's an STD, so it would get passed around the court, it causes madness if untreated, and the treatment wasn't discovered until the 20th century: Salvarsan.
 * A mysterious food, drink or drug reserved for royal use, with side effects.
 * They're Just Nuts: anything not covered by the above.

Whatever the reason, your rulers are bonkers, at least as far as objective outside observers are concerned.

Note that royal/imperial insanity is Truth in Television often enough that it can be a bit frightening.

The Caligula is a singular example of this trope, leaving out the familial tendencies, although they arguably applied to him too. In the Blood doesn't apply only to royals, but is one of the many reasons why a royal family can have recurring madness problems.

Anime and Manga

 * Eiichiro Oda's pirate manga One Piece has the World Royalty, otherwise known as the Tenryuubito (or "Celestial Dragons"). In particular, we meet a family of three World Nobles, St. Rosward and his children, St. Charlos and Princess Guu(not that one). Their only claim to influence is being the direct descendants of kings who founded the One Piece-verse's World Government. However, that claim allows them to perform atrocities ranging from shooting people for having the gall to speak to them directly to randomly naming women on the street as the newest additions to their long lines of wives (and sending the ones they're tired of back to the common folk) with absolutely no fear of retribution as the government will sic a Marine Admiral, complete with fleet, on anybody who dares defy them. Not that Luffy cared.
 * In Code Geass, the Royal Family of the Holy Empire of Britannia are all pretty messed up for the most part, ranging from Lelouch and Schneizel, to Cornelia, to Clovis, who wipes out a whole section of dilapidated city to cover up his mistakes. And that's not even talking about Emperor Charles, his . Luckily there's some hope. Euphemia is a generally sweet girl, and so is her younger sister, Nunnally. Crown Prince Odysseus is also pretty decent, though that may be because he doesn't really do much of anything (not to mention being probably the least intelligent of the family). Unfortunately this series likes to kill the kind ones.
 * An interesting variant: the heir presumptive of the Holy Kingdom of Saillune in Slayers, Prince Phillionel, while somewhat of a Leeroy Jenkins-like lunatic with a passion for physical fights, is probably the sanest and respectable member of the family and a stellar runner of the country; his younger daughter Amelia has most of his traits. The rest of the family is filled with Dirty Cowards who will do anything to move up a spot for the throne, with no concern for others - both of Phil's younger brothers, Christopher and Randionel, and his nephew, Alfred, have attempted to murder him. Alfred in particular contracts one (two in the anime) Mazoku to both take out Phil and take Saillune for himself while letting Christopher (his father) take the blame..
 * Saiunkoku Monogatari. Where to even begin? The old emperor deliberately pitted his sons against each other, and then exiled the one he favored. The princes all killed each other in a civil unrest that nearly destroyed their country. Years later, the youngest and least favored son is now the ruler, and has been spreading rumors about his sexuality to keep from having to produce an heir. He's also been Obfuscating Stupidity just so he doesn't have to rule. That only lasts until the royal advisers decide to get him a wife, who is another story in and of herself, seeing as she'd rather be a royal adviser than a consort. Oh, and that exiled prince? Turns out he's not so exiled after all... And that's not even getting into the seven OTHER families that make up the ruling class of Saimono.
 * Vampire Game is all about one princess's dealings with her own extended Royally Screwed-up family, wherein Incest Is Relative is the least screwed-up thing one can encounter, not to mention the Chimeras and of course Royal espionage. Oh, and there's a vampire who wants to kill her, too. This is mostly a comedy.
 * From the second season of Black Butler, Alois Trancy.
 * In Samurai Pizza Cats, Princess Vi is a selfish, spoiled brat who exiles people to Prisoner Island at the drop of a hat, her mother considers firing a rocket launcher at her daughter as an appropriate family greeting, and Emperor Fred... to say he's got a few screws loose is to imply he's got any screws left.
 * The Zabi family from Mobile Suit Gundam put the Screwed Up in Big Screwed-Up Family. Patriarch and Sovereign Degwin is a Well-Intentioned Extremist who seeks to Take Over the World in the name of his ideology. Eldest son Gihren is a psychopathic Social Darwinist who could not care less about ideology and just wants to increase his own power whatever the cost. Second son Dozle is more or less normal, but turns into an Axe Crazy berserker when turned loose on the battlefield. Daughter Kycilia is a cold-blooded amoral schemer who wants the throne for herself, and doesn't give a damn about human life. The only exceptions seem to be youngest son Garma, and Dozle's daughter, Mineva, who.
 * The Zabi family from Mobile Suit Gundam put the Screwed Up in Big Screwed-Up Family. Patriarch and Sovereign Degwin is a Well-Intentioned Extremist who seeks to Take Over the World in the name of his ideology. Eldest son Gihren is a psychopathic Social Darwinist who could not care less about ideology and just wants to increase his own power whatever the cost. Second son Dozle is more or less normal, but turns into an Axe Crazy berserker when turned loose on the battlefield. Daughter Kycilia is a cold-blooded amoral schemer who wants the throne for herself, and doesn't give a damn about human life. The only exceptions seem to be youngest son Garma, and Dozle's daughter, Mineva, who.

Card Games

 * Ironically, this pops up in Legend of the Five Rings. Odd for two reasons: One, the new family line had two generations before being wiped out in their entirety. And two, none of them seemed to be genetically crazy, the first emperor went nuts after being kidnapped and tainted, the next because he had way too much magical power, and the third because his sister died, and the evil of the world showed up and asked if he could join the royal court.

Comic Books
""All those years of dipping from the same genetic pool caused a wrinkle, a flaw in an otherwise normal family line. We set out to keep ourselves above the common man and found ourselves with a thing from the deepest pit of the Sith.""
 * Marvel Universe character of antiquity Namor the Sub-Mariner is both an example and a subversion. By all accounts, he rules the kingdom of Atlantis relatively well. However, he is also provably crazy: his unique Atlantean/Human physiology means that he requires both air and water to function properly, and if he goes too long without one or the other, his body chemistry drives him towards excessive rage and dangerous short-sightedness; a very dangerous thing indeed in someone strong enough to fight the Hulk to a standstill.
 * Also of Marvel, Magneto and his offspring formed a House of M in the miniseries of the same name, but in a subversion (aversion?), the royal family seemed pretty well-balanced. In the Ultimate line, however, the same 'royal family' is... Well, let's just say they've got problems. General explanation? Big Daddy M's crazy-genes, plus power-induced madness.
 * In the X Wing Series, royalty of the planet Eiattu interbreed and use technology to keep the line "pure" of the ills afflicting the common folk. But nature abhors a vacuum. Plourr Illo, revealed as the last confirmed survivor of the main royal family after the other nobles had a bloody revolution(her story was loosely based off of the legend of Anastasia), tells the other characters why the man rumored to be her brother(a new kind of revolutionary, this time of the common people) can't be him.


 * Well, she also knows it's not him because on the night her family was killed, her father managed to get the two of them out and her brother started screaming for the revolutionaries to come and find her, slit her throat so he could be Emperor. So she killed him.

Fan Fiction

 * Let Them Fade is a terrific Harry Potter fic exploring, in the form of a conversation between Snape and an adult Hermione, the results of long-term inbreeding among Purebloods, the Wizarding world's analogue to royal inbreeding: "For every Pureblood child in my generation, I have calculated or deduced the existence of five stillbirths or miscarriages."
 * Another Potterfic, Wish Carefully, depicts a Wizarding Britain where Voldemort and the Death Eaters thought they had forced a victory over Harry Potter and the side of the Light -- only to discover among other unexpected consequences that only Purebloods remained in England, and viable births were plummeting dramatically...

Film

 * The Prentiss family in The Manchurian Candidate (known in the novel as Iselin). The novel alludes pretty frankly to, and relates with equal candor at least one instance of same between All three are driven, passionate patriots working at high levels of office—Tyler was a diplomat, Eleanor is a Senator and Raymond is a Representative running for Vice President. Over the course of his campaign it is revealed that his mother

Literature

 * Lois McMaster Bujold's Vorkosigan Saga has the Vorbarra Imperial line. Thanks to inbreeding and genetic damage caused by environmental factors, some of the Vorbarra rulers have been... problematic:
 * Mad Emperor Yuri killed off most of his own family and then got dismembered and scalped by his own nobles, led by his brother in law/cousin.
 * Yuri's brother in law/cousin/successor Ezar was a relativly sane Chessmaster, but was also ruthlessly amoral beyond belief. The man signed off on a pointlessly agressive war he knew Barrayar would lose to topple his political enemies and kill his own son.
 * Ezar's son Serg was a twisted sadist who probably would have destroyed the Imperium if he'd been allowed to take the throne. Ezar killed him in a Uriah Gambit (too bad about the grunts).
 * Serg's son Gregor inherited the throne at age five when Ezar died, and, remarkably, grew up sane and stable thanks mostly to his adoptive parents, Aral Vorkosigan and Cordelia Naismith Vorkosigan. But Gregor became so paranoid about the genetic insanity in his family line that he refused to consider marrying anyone even distantly related to him. Since that equated to all the nobility on the planet, there was no clear line of succession, and Gregor's death would have caused a massive and probably final civil war...this posed a bit of a problem.
 * Fortunately Barrayar has recently gotten a handle on genetic engineering, eased up on the social stratification, and annexed another planet with its own unrelated set of merchant nobility (one of whom Gregor eventually married), so that nasty strain of nutjobbus maximus is likely to be cleansed from the line in the future. Much to the relief of Gregor, Aral, Cordelia, and every planet anywhere near Barrayar.
 * Cursed royalty also appears in Bujold's Chalion books (she seems to like the trope.) The main curse of madness/unluck/sterility/whatever-is-least-convinient in The Curse of Chalion is particularly nasty, in that it automatically spreads to anyone who marries into the family, making it completely impossible to eradicate without, as it turns out, direct intervention from the local gods. And then there's that strange familial wolf-madness thing in The Hallowed Hunt, too.
 * The unlucky king Orico tried to short-circuit this by but it probably wouldn't have worked anyway as the gods seemed pretty certain that you could not get out of this curse by clever trickery. It took a miracle.
 * Present in David Eddings' Belgariad, in the form of the royal line of Cthol Murgos, the Urgas family, with its hereditary insanity. In sequel series The Mallorean, the eventual successor to the throne is more or less sane, which makes sense, given that he's That's one effective way to get the crazy genes out of the royal line.
 * Made even more effective by the traditional method of ensuring easy succession: whoever gets the throne has every other potential claimant assassinated. Legally. Maybe the Murgos have had problems like this before...
 * Truth in Television—the early Ottoman Empire tried to cut down on succession wars by having all male relatives of a newly crowned sultan put to death. Predictably this only increased the number of succession wars, as every potential claimant to the throne knew that upon the death of the old sultan he had to either win the crown or die. Urgit's quote of "It was either the throne or the block." in King of the Murgos is drawn straight from history.
 * The various Tolnedran imperial dynasties tended towards this as well. Typically the first few emperors of a dynasty would be clever, competent men, but after several generations of inbreeding the line eventually devolved into rulers who were insane, imbeciles, or both. And then subverted by the Borunes, who by their contractual obligations have to marry Dryads. Introducing exogamy into the family line every generation must help. Of course, female members of the Borune family are Dryads also (and exclusively Dryads, there's no such thing as a female half-Dryad)...
 * In Teresa Edgerton's Celydonn trilogy, it is revealed in The Grail and the Ring that Mochdreff has been politically unstable for centuries largely due to the land having been cursed due to the sins of its last ruling prince. He committed an action so terrible that every single member of his family changed their names and refused to take up the sovereignty - although only people like Dame Ceinwen remember even that much of the story, and nobody remembers the specifics. Ever since, there have been Lords of Mochdreff rather than rulers styling themselves princes, until finally, due to the lack of a clear heir to the previous Lord, Prince Tryffin was appointed Royal Governor and took it upon himself to try to clean up the matter once and for all by getting to the bottom of the curse.
 * In the Sword of Truth series, the Rahl family line, for several generations, have been warmongering psychopaths. is, depending on the reader, either an exception, or adhering to the rule.
 * Everworld provides a variant: due to the royal tradition of Brother-Sister Incest, the last twelve Pharaohs of Everworld Egypt have all been mentally disabled and unable to rule in anything but name. This, coupled with the fact that the Egyptian gods are basically, made the country weak and unstable enough for the Amazons to take over.
 * In P.C. Hodgell's Chronicles of the Kencyrath, the house of Knorth, from which the ruling Highlords come, has what appears to be an inherited tendency toward madness. Torisen, the current Highlord, is terrified of what lurks in his bloodlines, and of becoming like his father and grandfather. Inbreeding and deliberate breeding for Shanir (magical) traits is probably responsible.
 * Redwall's Marlfoxes. The mother Silth is a raving maniac, her youngest is a sneak who deliberately feeds her mother's paranoia in order to weasel (or fox?) her way into power, and the oldest six offspring are just plain nasty to various degrees.
 * The Kingdom of Delain, in The Eyes of the Dragon by Stephen King, suffers from this problem every now and again. Particular mention is made of Mad King Alain, who was truly a raving and unstable lunatic but did his people the favor of dying quickly—he decided to go outside and play games on the lawn during a raging thunderstorm (lunatic, remember?) and got struck by lightning.
 * The Argaven kings of Karhide in Ursula K. Le Guin's The Left Hand of Darkness are described as congenitally mad. This seems to be accepted as part of the nature of kings on Gethen.
 * The page quote comes from A Song of Ice and Fire, in which the royal Targaryen line is blessed with greatness as much as it is cursed with madness due to centuries of inbreeding. It started with the first Targaryen king, who was a great man but unfortunately married and had children with both of his sisters; from there on out it's been a crapshoot. The line has produced many able warriors, statesmen, and scholars as well as a rogue's gallery of tyrants and psychopaths. Some Targaryens begin quite noble and lose their grip on sanity as they age, such as King Aerys II—by the end of his reign, he was known as King Aerys the Mad, and in the end his excesses sparked a revolt that toppled the dynasty. Daenerys, the only POV character with Targaryen blood (so far as we know) seems to have come out fine; her brother, Viserys... not so much. The books do give us some other normal Targaryens—Rhaegar (universally loved, killed in Robert's Rebellion), Maester Aemon of the Night's Watch, and
 * The Lannisters seem to be heading the inbred-madness route too:  are the product of Brother-Sister Incest between  Cersei is a paranoid schemer who eventually engineers her own downfall, and  her son was sadistic and unstable and had to be put down by . Hopefully averted with Prince Tommen and Princess Myrcella, who are both perfectly sweet children... for now.
 * In the Inheritance Cycle, there once was a King by the name of Palancar who tried to wage war numerous times with the Elves, even though every invasion was a hopeless crusade. Eventually his nobles rebelled against him to end the madness and had him exiled into a valley that later inherited his name. The Protagonist and his cousin, and the village they grew up in, descended from Palancar. Paolini so far has acted like that's at least somewhat of a good thing. Then again, he's also trying to convince us that the one who's really Royally Screwed-Up is Galby.
 * Fiona Patton's series Tales of the Branion Realm is set in a fantasy Britain where the gods take an active interest in their followers. The royal family, whose head is called the Aristok, is literally touched by the gods—the sovereign is the avatar of the Living Flame, a deity/demon/primordial critter which is a sort of symbiotic parasite. This makes the Aristok something of a cross between a hereditary Christ-figure and the real British system of the monarch being head of the church. Not only does the Aristok have divine right, she can prove it. Obviously, however, being the physical sacred vessel-on-earth of a fire god is bad for your health. Out of forty-one monarchs, fifteen have died in childhood, been assassinated, or committed suicide, and most of the rest went insane or died young. Three even converted to the Catholic-analogue faith, which makes for real cognitive dissonance among their followers as well as themselves. Clearly, whether this system is a blessing or a curse on the royal family is up in the air.
 * The first two of Mervyn Peake's Gormenghast books are, among other things, a long examination of this trope - the Groan lineage and their staff are a bunch of depressed lunatics, their spirits both crushed and perversely sustained by the castle and its ancient, messed-up rituals. They are a sympathetic bunch though - the melancholy and bookish Lord Sepulchrave and his unloved, Cloudcuckoolander daughter Fuchsia must surely be among the most tragic literary woobies of the last century.
 * Averted in some Discworld novels. Arguably, rulers of the city of Ankh-Morpork in the Discworld have had a tendency to be raving psychopaths. This applied when the city was ruled by kings, and was still true afterward when the kings were killed and replaced by the Patricians. By some stroke of astounding good fortune, the city is currently possessed of a Patrician, Vetinari, who is remarkably sane; as insurance, the rightful heir to the line of kings is also hanging around the city, and he too is remarkably sane, for certain values of sanity. How that happened is anyone's guess, as Ankh-Morpork is a thoroughly crazy city and tends to produce various types of craziness in anyone who lives there too long. Most of the citizens get used to it, though.
 * Interesting Times plays this fairly straight with the Emperor and Lord Hong. While the former's insanity is suggested to have a dash of genetic inbreeding behind it, the book hints that breeding the most paranoid, heartless, and evil bastards intentionally, and then not telling them cruelty is bad, may have been more of a problem.
 * Vetinari might seem to be more accurately described as effective. rather than sane, but The Discworld Companion describes Vetinati as "very, very sane". It is worth considering, however, that Jeremy Clockson and Sane Alex are also sane. Sanity and "normality" are not synonymous, and may be mutually exclusive.
 * In Thief of Time, someone mentioned sanity was 'defined by the majority'. Given how the majority of Ankh-Morpork act...
 * Played utterly straight when one book describes the lineage of kings in other Discworldian cities, and cites the last King of Quirm as "having been so inbred he repeatedly tried to mate with himself," or something along those lines.
 * In Jack Vance's Lyonesse trilogy, the king of South Ulfland's single son, Prince Quilcy, is feeble-minded and spends his days playing with fanciful doll-houses.
 * In the Tortall books by Tamora Pierce, the Copper Isles royalty tend to have madness crop up now and then, including one Princess Josiane. A character phrases it thus: "There's bad blood in the Copper Isles kings. They birth a mad one every generation. Josiane's uncle is locked in a tower somewhere. It comes from being an island kingdom- too much inbreeding." It turns out in further novels that it may not be just one per generation...
 * Two per generation, as of the Trickster books. The old king who dies and prompts the Succession Crisis and his brother who was mentioned as locked in a tower somewhere, and Josiane and Imajane among the old king's kids.
 * The Jimajen line might also have bits of this, though we only see two members: Rubinyan, whose only major flaws are an overdeveloped sense of honor and an inability to control his insane wife; and Bronau, who is extremely egotistical and ambitious without much common sense to go with it. Big brother is also ambitious as hell, but much more sensible...
 * Emperor Ozorne of Carthak and that cousin of the Tusaine line who starts the Tusaine-Tortall war in the second Song of the Lioness book both count.
 * Duke Roger, nephew to King Roald in the Song of the Lioness quartet, wasn't insane to begin with, but coming back from the dead (or not, precisely, if you believe him) certainly screwed with his head.
 * In the Tamír Trilogy (The Bone Doll's Twin, Hidden Warrior, and The Oracle's Queen) hereditary madness has hit the royal line. What makes this particularly dangerous is that the country's god has declared that only women of that bloodline can become ruler... or else. At the end...
 * To be fair, it could be that  just took after Daddy more than Mommy in that regard. The royal madness seems to have begun with Agnalain II. Let's not forget all the ways in which   And finally,
 * Unfortunately, the royal madness wasn't the only thread dropped in the rush to conclude the book.
 * Averted in the Honor Harrington series with the Star Kingdom of Manticore. Aside from being a constitutional monarchy, which limits the potential damage, Manticoran monarchs and heirs apparent are specifically prohibited from marrying members of the aristocracy. Aside from the "keeping in touch with the common folk" goal, it also removes the problems of inbreeding.
 * He swiped this from E. E. "Doc" Smith's Family d'Alembert series; under the Stanley Doctrine, nobility could marry commoners, but royalty was required to marry a commoner. While this helped, this was not totally successful in keeping loonies from the Imperial Throne (granted, the case of Empress "Mad Stephanie" could have been situational rather than genetic.)
 * In the case of the Stanley Dynasty, it may be as much cultural as genetic. At one point the competent, sane, and decent Emperor William (who is definitely an exception to the run of his ancestors) makes a joke about his and his wife's decision to abdicate at his age 70, so their daughter Edna "won't have to kill us." Edna is horrified by the joke, but her father points out that if he'd been more grasping, more determined to hang on to power forever, his child might have turned out different too (Edna is decent), because, as he notes, 'like begets like'.
 * Keith Laumer's Worlds of the Imperium briefly mentions the British royal family likewise adopting the policy that the Prince of Wales must marry a commoner.
 * The author even points out that if the Monarch is really bonkers, Impeachment is in the Constitution, with Parliament choosing the new Monarch from any person in the Kingdom.
 * And before being added to the official line of succession, the Monarch's offspring have to pass a psychological, and intelligence evaluation.
 * The Andermani Emperors on the other hand are competent but sometimes strange... from the start: the first emperor thought he was a reincarnation of Friedrich der Grosse (Frederick the Great of Prussia). Another was dethroned when he not only talked to his prize rose bush but also tried to make it chancellor.
 * And he was deposed by his own sister, who, while generally considered the best Andermani Emperor ever, had to legally declare herself a man, due to salic law practiced there. What it did to her mental state we could only guess...
 * It's hinted the reason why they are so successful is both the Insanity and the Genius of the line together. After all founding a New Prussian Empire on a Chinese colony world and making it into a regional power does sound pretty nuts. The first Emperor was a rich space pirate who saved the world from starvation.
 * The Civil Government of the planet Bellevue in The General Series is Royally Screwed-Up in that both the current governor and his acknowledged heir are borderline clinical paranoids, and becoming less borderline all the time... They're both intelligent and competent. Just envious/jealous and distrusting. Really, really distrusting.
 * The Ancient and Most Noble House of Black from Harry Potter. It has a long reputation for its members being insane, lots of inbreeding and are pureblood (what many consider royalty in the Potterverse). This also applies to most pureblood wizarding families in general (with a few exceptions)... who have, incidentally, all probably intermarried with the Blacks.
 * The Gaunts are even more messed up, and possibly more inbred, since Dumbledore mentions that they're the only descendants of Salazar Slytherin left. Ironically, their worst member comes when they finally manage to get some new blood: Lord Voldemort, the son of Merope Gaunt and a Muggle.
 * The Raiths in The Dresden Files, the royal family of the White Court of vampires. The White King is a Complete Monster who rapes his female children into supernatural slavery and kills off his sons. His daughter Lara is a Magnificent Bastard who lives on the line between Heroic Sociopath and Friendly Enemy Anti-Villain. The only reason his son Thomas lived to adulthood is by playing the Rich Idiot With No Day Job card for everything it's worth. The only one who doesn't appear to be incredibly messed-up is.
 * In The Silmarillion, the first king of the Noldor, Finwe, is a good man, but after his death, the crown goes to his eldest son Feanor, who was very paranoid before, and became completely crazy (if still very charismatic) when his father was murdered. Once Feanor is also dead, his son Maedhros should become king...but he averts this trope: knowing how dangerous the Oath he and his brothers have sworn is, he abdicates and lets his uncle rule. It's a wise move: the sons of Feanor do commit some horrendous acts, and the two eldest eventually become insane, but at least they only rule a fraction of the Noldor.
 * The Bible is chock full of lousy or downright ax crazy evil kings of Israel who choose to snub the God who saved their ancestors from Egypt, so much so that the good kings are the exception.
 * And even the good kings still tend to be royally screwed up. Witness David, whose punishment for committing adultery with Bathsheba and having her husband killed was that his first son by her fell ill and died a week later, and the rest of the sons started killing each other for various reasons. David ultimately appoints Solomon as his successor, and even then the succession crisis doesn't end. Solomon was also messed up in his own right on account of his harem of foreign wives.
 * Averted in the Heralds of Valdemar series, in part because Valdemaran law forbids a monarch (or, presumably, heir) from marrying anyone within two degrees of kinship. And the newly-crowned Selenay plays it to the hilt to keep her councilors from forcing her into marriage, too. The requirement that all monarchs must be Heralds is probably more important—there's nothing saying a Herald can't be a bit nuts (Hi, Lavan and Vanyel!), but at least it's the type of nuts that doesn't result in the abuses seen on the rest of this page.
 * King Rodric IV in The Riftwar Cycle. Hated and abused by his father for being a sickly runt, he proved to be as sick in mind as he was in body. Apart from using openly about how his power would allow him to randomly pick out random people and have them executed for no reason other than he wished to see them die, he squandred much of the tax revenues of The Kingdom Of The Isles on a series of aesthetic public works programs designed merely to make the city of Rillanon look prettier. Worse still, he denied vital military aid to the Western half of his Kingdom, fearing that the soldiers would be used to build an army against him, which helped to drag the first Riftwar out for the better part of a decade.

Live Action TV

 * In Babylon 5, the nephew of Emperor Turhan, Cartagia, became the Emperor of the Centauri Republic after his uncle's death. Emperor Cartagia was as bad as any fiction-version of Caligula, and apparently modeled after him.
 * One episode of Doctor Who strongly implies that eventually the British Royal family might become werewolves.
 * But merely because Queen Victoria had been scratched by the werewolf at Torchwood manor and infected by an alien blood parasite that used human bodies as hosts.
 * Referenced in the Red Dwarf episode Rimmerworld. Kryten explains the problem of having an entire society descended from Rimmer by drawing comparisons with European monarchies of the 19th and 20th centuries. The actual leader of the planet seems to be an example himself.
 * A lot of tension surrounding the Succession Crisis in Merlin is based on this trope. It's revealed in series three that the Pendragon men have a history of mental illness, one which King Uther ultimately succumbs to after his betrays him and takes the throne.

Mythology

 * The entire Greek Pantheon, in many ways.

Tabletop Games

 * Warhammer Fantasy Battle is fond of this trope. During the most decadent period of the Empire's history it's implied that inbreeding reached epic proportions and led to actual mutations among the nobility. They seem to have straightened things out for the most part by the "present day" though. Thank Sigmar for the witch hunters, eh?
 * Subverted in the case of Bretonnia though, where it's the peasants who are inbred and deformed. Some recent anthropological research suggests this may be Truth in, er, roleplaying games.
 * Warhammer 40,000 has numerous examples of hereditary planetary rulers who follow this trope - though Inquisitor Vail would point out that this doesn't happen quite as often as the stereotypes would have you believe. In worst case scenarios (such as Osric the Loopy, mentioned in passing in The Traitor's Hand), the Officio Assassinorum can be dispatched to "tidy up" matters.
 * ...actually, that's a lie. In worst case scenarios, the Royally Screwed-Up ruler is a heavily mutated Chaos-worshiper who unleashes the Legions of Hell on the planet they're supposed to be governing. In those scenarios, stronger steps are taken.
 * And that's not getting into The Emperor, his sons, and the tragedy that shattered the galaxy.
 * Of course, the Emperor was probably the sanest person in the galaxy and up until the Heresy the majority of his sons were perfectly sane as well. (The exceptions being Kurze, a murderous sociopath, Angron, a blood-crazed berserker, Lorgar, a raving zealot, and Alpharius, who had colossal delusions of grandeur)
 * In the Old World of Darkness RPG Werewolf: The Apocalypse, many of the ruling tribe, the Silver Fangs, suffered from this—despite the fact that werewolves had to outbreed (werewolf-werewolf matings were lucky if their children were just insane). Of course, interbreeding with the Habsburg line didn't help.
 * Somewhat justified in that the Silver Fangs had such an obsession with lineage that they refused to breed with any humans that weren't royal. So they managed to get most all the bad traits of just about everyone in the "Real Life" section below.
 * From BattleTech, the Liao dynasty of the Capellan Confederation seemed to produce only two kinds of rulers. Batshit insane and batshit evil. The non-insane non-sociopath Liaos tended to defect to other nations, neatly removing any descent chance of cleaning the batshit out of that genepool for a long time. Fortunately for the Capellan Confederation, the batshit evil variety tends to serve the needs of the populace; they're seen as evil by other nations that they're screwing over.
 * Also, the Steiner dynasty of the Lyran Commonwealth has a genetic predisposition to a few psychological conditions. They never get to the truly batshit level of insanity, but they have been known to do substantial damage to their nation.
 * It's applied to each of the five major factions at some point. Perhaps the most notorious version in-universe is Jinjiro Kurita, who ordered his troops to kill 52 million people on a world where his father was killed by a sniper.
 * The House of Naelax, rulers of the Great Kingdom of Aerdy in the Greyhawk setting, were commonly viewed as being possessed by demons. This article, although written by a fan for his own campaign, is nonetheless a good summary of what the Ivid Overkings were like.

Theatre

 * By the end of Electra, Chrysothemis is probably the only member of the royal family who hasn't tried to murder another member in retaliation for a previous murder.

Video Games

 * Armed and Dangerous has an interesting case: a magical curse cast on the kingdom of Forge causes one king of the country to be a clever Evil Overlord, and his immediate successor to be a kind-hearted dimwit, and his successor again to be an Evil Overlord, and so on. In retrospect, it might have been a better idea to make the evil one the idiot.
 * The Dresari family in the Mechwarrior 4 series appears to suffer from this; it's doubly painful because . Per a previous example, this is not entirely uncommon in the BattleTech universe.
 * Weirdly enough, Word of God retcons this saying that the latter incident mentioned above is in fact
 * Fire Emblem. Good lord, Fire Emblem. Every freakin' game. Granted, no more than two games (except 1/11, 2 and 3) take place in any one continuity, but regardless, there is at least one mad ruler per game, or at the very least, mildly evil (Blazing Blade's King Desmond wasn't really mad, just a petty idiot—and Marquess Laus wanted to rule all of Lycia, but never actually did.) Well, okay, Radiant Dawn actually had a bunch of evil senators trying to usurp the empress of Begnion and an Evil Chancellor at the side of the new king of Daien...FE10 did have Naesala, but he turned out to be...compromised.
 * Zork - the entire Flathead dynasty, ruling or not.

Web Comics
"Vole: De pipple of Mechanicsburg would not ekcept dot [killing Castle Heterodyne] as proof dot she iz a Heterodyne...not unless she danced nekked though de ruins vile trying to shoot down de moon--turned all de tourists into monsters--and den built a very dangerous fountain out of sausages."
 * Nearly every clan in Drowtales could fit into this, but the Sharen are the most screwed up. Matricide, starting a civil war, and subjecting one's entire clan as well as any female summoner to demonic Tainting, is a good start for proving a case of mental imbalance.
 * The Sarghress clan apparently has a history of child abuse. Allegedly, Quain  when Mel refused to bear an heir for the clan; in turn, according to a non - canon side story, , and Syphile once locked Ariel (who was physically about 5 years old at the time) and Fuzzy (Ariel's cat) in a cell with no bathroom for a week, and then
 * Kharla, ruler of the Vloz'ress clan, is the page image for Living Doll Collector.
 * This problem is endemic in Girl Genius. Sparks, being creative geniuses, naturally respond to any intellectual problem or technological innovation with "ooh, shiny!" The powerful ones also tend toward considerable charisma and psychological instability. Throw in a lot of "manifest destiny" and "right to rule" noble sentiment, probably lifted from real-world history, and you get feuding warlord dynasties unleashing war machines and fearsome monsters upon one another constantly. Anti-Villain Baron Klaus Wulfenbach forged a Pax Wulfenbach of sorts, but there's still a fair amount of scheming and rebellion against the (perceived) Evil Overlord. Even the heroic Sparks, mainly heir to legendary heroes Agatha Heterodyne and her probable love-interest/only viable political rival, Klaus's son Gilgamesh Wulfenbach, are prone to manic episodes of creativity and occasional violence.
 * The House of Heterodyne, of which Agatha is the only known living member, deserves special mention. For generations, they were the most insane and dangerous maniacs that the world had ever known, and also some of the strongest Sparks. The previous generation, Agatha's father and uncle, are an exception, having used their brilliant insanity for good; but according to one observer, the people of her hometown would accept a crazy Heterodyne as legitimate:


 * Add in the fact that the Hetrodynes were the ones who created the Jagers (think WW 1 Germans fused with Orkz and muppets) and they were plenty messed up too. Basicly, the Hetrodynes bred right past crazy and back around to normal.
 * Add to this the House Sturmvoraus, apparently affected with an inborn Chronic Backstabbing Disorder, and a Prince in love with The Other.
 * Princess Sara in 8-Bit Theater is smart, sexy, and sane enough to fully realize her father is a Cloudcuckoolander with genocidal tendencies. Naturally, she doesn't hold much stock in hereditary rule. She's still a rude, shrewish sociopath, though, and engineered her own kidnapping.
 * How bad it is: No matter what horrible evils she unleashes on the populace when she comes into power, it will look like a golden age compared to the completely ruinous and unhinged chain of decisions King Steve makes every day, simply because she's not enough of an idiot to be capable of the same levels of casual destruction.
 * The Masters Royal Family of Chess Piece are said to be cursed. Luckily, it skipped a generation. Unluckily, the current Prince has seriously planned on taking over the world since he was four.
 * In Homestuck, the Highbloods (high-ranking members of the troll caste system) seem to be innately prone to psychotic behavior. (Well, more so than the rest of the species.) The highest bloodtype; the Imperial, or Tyrian line, boasts Her Imperious Condescension, a millenia-old tyrant known for her cruelty and fickleness. (Interestingly, her descendant Feferi seems to be much more benevolent, making this part a possible subversion.) There's also the Grand Highblood, a warlord who often killed and mangled people for the hell of it, and  Equius, a noble-ranking blueblood, has some peculiar anger management issues and pretty much states up front that highbloods are just genetically predisposed to violence and psychosis.

Web Original

 * Zeus and Hera and their children in Thalia's Musings, ranging from good but troubled (Apollo and Artemis) to flat-out crazy (Eris).

Western Animation
""What an amazing lie that was! The other nations hate us!""
 * The line of the Fire Lords in Avatar: The Last Airbender has... issues. Specifically, a tendency toward being sociopathic and homicidal on both a personal and national level. Again, if there's hope for stopping the ruling lunatics, it seems likely to come from the branches of the tree that didn't get hit with the crazy stick - disgraced traitor Iroh, or screwed-up-but-trying-to-improve disgraced traitor Zuko. For the most part the Fire Lords seem to have avoided taking out their issues on their own people, so their own common folks seem to be reasonably pleased with their rulers. It's just everyone else on the planet who's rightfully terrified. The problem isn't likely to go away until the planetary balanced is fixed; it seems to be spiritual in nature. (One ancestor went power-mad three generations back, and his successors have continued his policies. And why not, as they seem to be working fine - as long as you're Fire Nation, that is.)
 * At least Zuko realizes this.


 * For what it's worth,  things looking up.
 * Many viewers suspect that Zuko, though it didn't help his kid sister Azula much, or did it? Perhaps she could have been worse.
 * That one sibling spent three years with Iroh while the other stayed with their bastard of a father may also be a factor.
 * The family being just that screwed up was lampshaded when Azulon demanded Ozai pay for his offense of trying to take Iroh's place as heir by killing his own son, and knowing the pain Iroh did. Living in the same palace, Azulon never realized that his second son actively despised his own elder child and would have killed him gladly at any time. Briarpatch, anyone?
 * Alternate theory is that he was going to take Zuko away from Ozai and force-adopt him to Iroh, his own favorite. (Ozai's issues, especially with Zuko, almost certainly stem in part from his resentment of Iroh.) Since the Fire Nation seems to default to primogeniture and the cadet-branch system, this would have put Zuko ahead of Ozai. It makes more sense from Azulon, and makes what happened with Ursa even more tragic, but is kind of a forced interpretation of canon.
 * Iroh himself tells Zuko that his internal conflict may have something to do with his double ascendancy. The mystery is: if Zuko is twisted between good and evil, why is Iroh the one who chooses good way before Zuko, when he has a pure evil ascendancy?
 * Iroh's good side is suggested, by Fanon at least, to have something to do with his trip to the spirit realm after.
 * Because he doesn't choose it before Zuko. Chronologically he does. But Iroh couldn't have been younger than his mid-forties when he chose good. Zuko was sixteen and already showing signs before that. Well, before Ozai temporarily fixed that, anyway. Speaking of which, what about Ozai scarring and banishing Zuko for talking out of turn? Daddy issues, much?
 * The Heinous family on Jimmy Two-Shoes, which has ruled Miseryville for centeries. You know something's wrong when Lucius VII is considered the least evil ruler the town's had.
 * Candy Kingdom Law, in Adventure Time, is "complicated," according to Princess Bubblegum. In the event that anything should happen to PB, who inherits the throne? Her maladjusted, overly-sensitive, socially awkward, most likely brain-damaged, insensitive, angry, sour-tempered, alienated son/science experiment gone wrong, Lemongrab. Arguably, Lemongrab is a pretty sympathetic example of this trope—he obviously has an... ahem, a delicate condition, but that doesn't stop everyone from despising his guts for being a jerkass most of the time and sending everyone to the dungeon for a million years.

Real Life

 * In Sweden, the Vasa line of rulers was known for this attribute. Gustav Vasa was a competent tyrant who united Sweden ; his first son Erik XIV was absolutely insane (killing servants who wore colorful clothes) which may have been because arsenic was added to his pea soup as a food colouring, the middle son Johan III was a brutal warlord who turned into a neurotic depressive; the youngest son Karl IX was a paranoid madman who ordered several massacres and a religious zealot who extirpated Catholicism off Sweden and insisted instilling Calvinism. The son of Karl IX, Gustav II Adolf was definitely competent and is still a national hero in Sweden, but also arguably a scary and megalomaniac Blood Knight. His daughter Christina was actually quite competent, if a little eccentric.
 * Let's not forget prince Magnus (brother of Erik, Johan, and Charles(Karl)), who was too mentally unhinged to even participitate in his relatives' infighting. Interestingly, letters show that the other three seems to actually have cared for the insane Magnus a lot - despite showing murderous hatred towards each other in other circumstances. On the other hand, he was never a threat to them...
 * There's also Sigismund Vasa, who somehow was elected king of Poland-Lithuania. While relatively mentally stable, he was stubborn to a fault, refusing to see Poland-Lithuania as anything else than a tool to get him the Swedish Crown, even though Sweden didn't want him, as he was catholic. His reign led to nearly constant war between Poland-Lithuania and Sweden and Russia for the next 100 years, which ruined the country and led to its eventual partition.
 * The Vasa line survived far longer in Poland after it had become extinct in Sweden. The Vasa kings of Poland represented themselves as the legitimate claimants of the Swedish crown.
 * Henry VI had some kind of mental illness which left him near-catatonic for long periods. It may have been inherited from the French royals; Henry's French grandfather, Charles VI, was also mentally ill, sometimes claiming to be made of glass. Charles's madness led to a civil war, and a English invasion - Agincourt and Joan of Arc; Henry's madness led to the War of the Roses - two wars which effectively purged the royal lines of madness. This shows that occasionally Real Life can be more sensible than fiction: most medieval kings had to be competent, or they got removed.
 * Much later there was George III, of the "talking-to-a-tree-because-he-thought-it-was-the-king-of-Prussia" style of crazy, but that was an isolated incident. His son, George IV, had to rule as regent for years while they waited for him to die. (These are the Georges that turn up in the Blackadder season 3 finale, incidentally.)
 * George III only developed madness in later life - earlier in life he was charming, handsome and reasonable well-adjusted. Recent theories suggest that this may have been due to a genetic condition called porphyria that is hereditary, it just tends to skip several generations without manifesting. May have entered the British Royal Family from the Scottish line James I and VI who may have inherited it from his mother Mary, Queen of Scots. Mary, James and George are just the only reigning monarchs to suffer, all the other possible instances just cropped up in branch lines.
 * Skeptics of the theory that George III inherited porphyria from either Mary and James have pointed out that that there is as yet no evidence of porphyria in the intervening generations of Elizabeth, Queen of Bohemia; Sophia, Electress of Hanover; George I; George II; and Frederick, Prince of Wales. Certainly neither Mary nor James displayed any signs of madness. In any case, George III's doctors tried to cure his madness by giving him mercury ("It works on syphilis, maybe it'll cure this too..."), which obviously didn't help.
 * Henry VIII started out as a good king, but became more and more cruel and egotistical as he got older (as many of his wives discovered). Theories differ on why, but illness, a sports career ending injury, and the lack of a backup male heir have all been put forward as helping him along the path from arrogant and short-tempered to paranoid megalomaniac.
 * The Habsburgs were inbred even by the standards of European royalty, which might not have been a problem except that their matriarch (Juana de Trastamara aka "Juana La Loca", known in old-timey English sources as "Joan the Mad") became a total basket case after the death of her husband Philip the Handsome (their marriage, fortunately for them and unfortunately for Spain, was Perfectly Arranged); she ended up incarcerated by her own father Ferdinand and, later, her son Carlos I/Charles V, who had to be told to treat his poor mother better as a condition to be elected Holy Roman Emperor. Don Carlos, the rebellious son of Philip II, was insane to the point of being physically dangerous and would take swipes at passing servants with a knife. Ferdinand II's favourite occupation was rolling around in the bin. Even the more mentally stable scions of the dynasty tended to feature a massively disfigured lower jaw, often to the point they could not even close their mouth.
 * The trope's picture is a portrait of Charles II of Spain, last Habsburg King of Spain—and the art style of that time tended to gloss over any blemishes someone had (much like fashion magazine photos today) so in all likelihood, his looks were even worse. He was physically and mentally disabled as well as disfigured (he had the "Hapsburg Lip" to such an extent that he could not close his mouth; that's why his tongue poking out). His subjects nicknamed him "The Bewitched". Unsurprisingly, he closed the Habsburg chapter in Spain by not perpetuating his genetic pool, constituted among many other issues by his grandmother being also his aunt. After all, Charles descended from Juana La Loca just 14 times... twice as a great-great-great grandson, and 12 times further. When all your ancestors are descended from "Joanna the Mad", you're not off to the best genetic start in life.
 * It should be pointed out, however, that recently a lot of historians are questioning Joanna's madness. Witnesses who weren't paid by the ones who wanted her throne claimed she had opened the coffin of her husband once (which was the custom at that time, to ensure the right person was being buried). Yes, her throne: technically she inherited it from her mother Isabella and was a queen in her own right, a fact that annoyed her father, husband and son equally. While she most likely was depressive and passionate, her "fits of madness" mostly broke out when her children were taken from her or when she was locked up for years. Who wouldn't have fits under such circumstances? (By way of comparison, England's Queen Victoria was morbidly depressed for decades after being widowed and avoided almost all official business, to the point where leading political figures seriously considered declaring a republic just so they could have a full-time head of state to rubber-stamp their decrees; nobody thinks of her as insane, just broken-hearted.)
 * Oh just go ahead and link to Charles II's sordid, warped 'tree' that doubles back on itself. The Habsburgs felt that not marrying fellow high royalty was beneath the worth of a true king. They were also devout Catholics, and half Europe went Protestant after 1521. Furthermore, the greatest Catholic country remaining was France, which hated the Habsburgs and constantly was at war with them. This meant that the Spanish and Austrian branches of the Habsburg family married each other a whole lot.
 * A borderline case is Emperor Ferdinand I of Austria. While probably not insane, he was definitely epileptic and had a hydrocephalus. His (rather euphemistic) honorific was "the kindly". The only direct and coherent command he ever gave during his reign was "I'm the Emperor, and I want dumplings!", upon being told that the apricots needed to make the kind he wanted were out of season. After being told by his chancellor that the people outside the palace were carrying out a revolution, his answer reportedly was: "Are they allowed to do that?" Since he remained childless, the defects did not get a chance to progress down the line. Not that the line would ever have become Emperors; after the aforementioned revolution, the government convinced him to abdicate in favor of his saner and (it turns out) more capable nephew Franz Joseph (who would reign until 1916).
 * In general, while the decision to marry the House of Trastamara, while political genius, was a bad idea genetically for the Habsburgs, considering that the Trastamaras were incredibly inbred in the pre-Reconquista period (with most of Europe writing off Spain as a Moorish territory, there weren't terribly many foreign royals to marry); one is reminded of Larry Gonick's take on the House: "I'm Pedro El Cruel! What can I do to you?"
 * The sad part? THAT WAS THE KING IN QUESTION'S REAL NICKNAME.
 * ... By his enemies. To his supporters he was "Peter the Lawful" and it has been pointed out that he was in fact a rather competent administrator and military leader and tolerant of religious minorities as well (he was in particular loved by the Jews). Naturally, being deposed and decapitated by your bastard half-brother will do that to your memory, even if you also had a penchant for decapitations.
 * It was not "the Lawful", but "the Justicier", because he liked applying summary justice (i.e. beheading people), mainly because he was surrounded by traitors.
 * The founder of the Trastamara line was in fact Pedro's sane illegitimate half-brother. That didn't keep the unfortunate recessives from popping up in subsequent generations. Somehow the Aragonese and Castilian branches of the Trastamara each managed to produce a sane and competent son and daughter to marry each other and unite Spain, something of a historical and genetic miracle.
 * Whether or not the Imperial Roman lines count is a matter of much discussion; certainly many of them were raving mad by our standards (particularly, well, Caligula..) For an entertaining view on how insane things got, check out I, Claudius. How much of that was due to the Emperors being corrupted by absolute power, or due to outside factors such as real organic illness (Roman plumbing was great in theory but may have resulted in lead poisoning issues in practice), or how many Emperors just seemed paranoid and vicious because that was the only way to survive as a Roman Emperor, is both debatable and debated. In any case, much later in the history of the Roman Empire, due to a lack of male heirs, necessity demanded that the Emperors begin choosing their successors and adopting them, rather than letting random genetics decide who should be in charge. The Roman Empire experienced a century-long period of stability and (relatively) peaceful growth as a result.
 * Then again, we have so few contemporary histories of 1st century Rome that we can't be absolutely sure that what we do have (Tacitus and Suetonius, who lived a century or so later, are our best resources) is accurate. The Romans weren't above exaggerating and lying to prove a moral point.
 * Given that of the 88 Byzantine (Eastern Roman Empire) Emperors only 29 died on natural causes, with at least a dozen dying at the hands of family members, I'd say that paranoia is indicated.
 * It was Augustus, the first emperor, who was obsessed with making an heir of his successor. Fairly quickly, the Julio-Claudians killed themselves out. Instead of reverting to a republic, it turned back into the military-power-based autocracy founded by Caesar. Adopting an heir was not a particularly novel idea (Romans could even adopt an heir in their will) but it was often a necessity, given that all it took for a coup was for the emperor to be unpopular with enough of the army (or alternatively, for someone else to be more popular with the army). Despite this, the occasional "good" emperor would become enamored of putting his blood-relatives on the throne. This almost always ended unhappily. (Right, Aurelius?)
 * Other heavy metal poisoned rulers include Ivan the Terrible (Mercury), Napoleon Bonaparte (Arsenic) and most wealthy people from about 1730 to 1815 (Lead in the Port and other fortified wines to make it sweeter, lead makeup, lead in the drinking water, mercury in the clothing, arsenic as a common green dye).
 * Queen Victoria was a hemophilia carrier, passed it on to three of her children, and the bad gene was spread to many other royal houses across Europe, royally screwing them over (including the Romanovs, whose last legitimate heir was doomed to die from this illness before maturity; that's why the faith-healer Rasputin gained such prominence). The extent of the hemophilia gene among royals became so universal that many commoners thought that hemophilia was the genetic marker of royalty. Even the 1950s B-movie Queen of Blood decided that the extraterrestrial featured must be royalty solely because she was a hemophiliac. That is one impressive little allele.
 * In a Real Life subversion, the nation of Nepal abolished its 240-year-old Royally Screwed-Up monarchy in 2008, becoming a republic. The Nepalese reached this decision after Crown Prince Dipendra allegedly went Ax Crazy in 2001 and gunned down most of his relatives, including his father; by law, Dipendra was crowned Nepal's king in turn, despite being both a mass murderer and comatose from a self-inflicted gunshot wound. Dipendra quickly succumbed to his injuries and his uncle was crowned king, but Gyanedra's abortive attempt to abolish Parliament (along with conspiracy theories that he had engineered the massacre) was the last straw: Nepal had had enough of this trope.
 * It also helped that Nepal was in a Civil War at the time with the Maoists, one of whoms demands was the abolition of the Nepalese monarchy.
 * Sometimes members of the the Ancient Egyptian family, (because it went on for quite a while) had birth defects such as elongated fingers and toes from inbreeding. Pharoh's would often marry their sisters, half-sisters and cousins.
 * Though many of the pharohs and royalty married their direct family members, that did not mean they actually had sex. They could have as many people on the side as they wanted, but it was simply fashionable to keep up the appearance of incest going on. Values Dissonance to the max.
 * Heir Club for Men says not quite—the lineage was best secured by a son of the king who had the maximum royal blood on his mother's side, so your sister's kid was the best bet. The dynasties and therefore the gene pools involved changed a good deal more than the OP indicates, though.
 * Everyone who studies the Middle East gets sick of hearing about Ibn Khaldun, because people drag him in where he doesn't really fit, since he studied specifically North African bedouins, but he established a generational rotation for nomads and the civilizations they conquer: the first generation is rough and tribal and not quite civilized, the second (with luck) still understands how to maintain what made his father strong but also has been brought up in the arms of city culture, the third begins to lose touch and grow soft, and after that they just fall apart until the next new dynasty rides in and replaces them. Rinse, repeat. This paradigm is adapted to all kinds of uses.
 * People drag him in where he doesn't really fit because his theory is fairly effective; the Islamic dynasties of Spain are an excellent example of it. And because he was one of the first historians to have a theory of history instead of just a straightforward record of what happened. You might get sick of hearing about him, but he's pretty important. (We Have Become Complacent on this very wiki is a possible relative of his theories.)
 * It's actually effective for the Mediterranean region in general, which is what Ibn Khaldun knew; while he mostly gave North African examples, you have to bear in mind that the Muqaddimah (which is where these theories come from) was actually the introduction to a much larger history of North Africa (muqaddimah just means "introduction" in Arabic). With the possible exception of the Roman and Egyptian empires, pretty much all every great empire of the Mediterranean region in the pre-modern era were founded by at best half-civilized conquerors who took over the established civilizations: the Akkadians, Assyrians, Hittites, Persians, Macedonians, Germans, Arabs, and Turks (to name only the most obvious examples) all did this. Even the Romans were pretty uncouth when they started taking over Italy; between their militarism, agrarianism, lack of high culture, and piety, the perception among the Etruscans and Greeks of the peninsula was essentially the same perception that American bicoastal elites have of Flyover Country. Even the Egyptians count to some degree, as their empire was only established after native rulers adopted the technology of foreign barbarians (the "Hyksos", who were probably Canaanite shepherds).
 * Ferdinand The Good (AKA Goodinand The Finished). While it's debatable whether or not he was retarded, he was epileptic.
 * Ludwig II of Bavaria was considered by many to be mad, and deposed because of it, followed by his death the next day of either murder or suicide. For the most part his 'insanity' consisted of an obsession with building elaborate castles, opera music, and beautiful men, and he's more fondly remembered now.
 * After that, his successor, King Otto, spent his entire 'reign' institutionalised.