Being a man with an extra Y chromosome is a pretty common genetic variation (1 out of 1,000 biologically male humans). Also known as Jacobs' syndrome, an XYY karyotype is currently believed to cause only two symptoms: above-average height and an increased tendency to learning disabilities (although only around half of XYY males show diagnosable learning disorders).

In fiction, however, being XYY makes you a criminal! It's simple logic:
* A Y chromosome makes you male.
* Therefore, an extra Y chromosome makes you [[TestosteronePoisoning more male]].
* Men are more aggressive and violent.
* [[InsaneTrollLogic Therefore, an extra Y chromosome makes you even]] ''[[InsaneTrollLogic more]]'' [[InsaneTrollLogic aggressive and violent.]]

This trope is based on a couple of widely-reported studies from the 1960s which didn't pan out on further research (see the RealLife section). Media, however, didn't get the memo.

This trope is likely to show up in crime dramas and other works that involve both bad guys and genetics researchers or genetic typing. It's usually either the reason the bad guy is bad or an aggravating factor in his badness.


* Deadfall, from ''Artemis: Requiem'', was artificially given an extra Y chromosome. His powers include super strength and the ability to manipulate testosterone. He is described as perhaps the most aggressive male on earth.
* In ''Deathblow: Byblows'', several clones of Michael Cray (Deathblow) are revealed. One of the clones, Judgment Cray, was given an extra Y chromosome and is a psychotic, violent killer.

* In ''Babel Fish'', an [[Franchise/XMen X-Men]] fanfic, a character is revealed to have an extra Y chromosome. Beast explains that this may be responsible for lankiness and lower IQ, but states that the notion that XYY males are prone to becoming serial killers is nonsense.
* The ''Series/{{Charmed}}'' fanfic, ''The Ultimate Demon'', features a character that has an extra Y chromosome. According to Memphis, another character, not only are XYY men prone to violent behavior but the 47,XYY karyotype may be due to demons and humans mating in the distant past.
* In ''Fourth Down'', a ''Franchise/FireEmblem'' fanfic, a new football coach wonders why Ephraim is so aggressive. Eirika suggests it may be an extra Y chromosome.

* ''Film/{{Alien 3}}''. All the prisoners at the penal colony are XYY males. Interestingly, the prison was actually shut down and these were the prisoners who elected to stay. Many of them claim to be trying to change as well.
* Referenced in ''Film/ScaryMovie 3''.
* The biology research institute in ''Film/TheCatONineTails'' is researching it while the murders happen around it.

* ''[[ The XYY Man]]'' began life as a series of novels by Kenneth Royce, featuring the character of William (or Willie) 'Spider' Scott, a one-time cat-burglar who leaves prison aiming to go straight but finds his talents still to be very much in demand by both the criminal underworld and the British secret service. Scott has an extra "y" chromosome that supposedly gives him a criminal predisposition - although he tries to go straight, he is genetically incapable of doing so.
* Played utterly straight in the ''Franchise/StarTrek'' short story "The Procrustean Petard": the ''Enterprise'' crew is gender-flipped by an alien device which doesn't know how to deal with HalfHumanHybrid Mr. Spock, so it makes him XYY instead.
* This trope is mentioned in ''Nightlines'', a novel by John Lutz.
* Mentioned in ''Succulent Prey'', by Wrath James White, in which a character states that the link between an extra Y chromosome and serial killers has been shown to be false.
* In ''Full Cry'', by Rita Mae Brown, two characters discuss criminals, with one claiming many criminals were “born bad,” giving as an example that most men who are born XYY can’t control their violent behaviour.
* In ''Gone for Good'', by Harlan Coben, Will wonders at one point whether Ghost, who is very aggressive and displays psychopathic tendencies, has an extra Y chromosome.
* In ''Missing Pieces'', a novel by Joy Fielding, a character is asked what causes someone to become a violent sociopath. He responds that there is no consensus as to whether it is more nature or nurture and cautions that attributing such behaviour to an extra Y chromosome is problematic.
* The short poem, "It's Easy As XYY", refers to this trope, linking an extra Y chromosome to violence.
* In "Sam, Soren and Ed," a short story by Guy Vanderhaeghe, Ed describes another character, whom he first sees aggressively practising martial arts, as an “extra-Y-chromosome type.” After a confrontation with his estranged wife, Ed almost has to fight the man, but the police show up and diffuse the situation.
* In ''Slow Motion Riot'', by Peter Blauner, Andrea refers to a character who seems to "love to do crimes" as acting like someone with an extra Y chromosome.
* This trope plays an important role in ''The Campus Detective: The Sociopath'', by Dave Larson.
* ''The Stranger in Goldrush'', by Sheila Bush, mentions this trope. A character briefly refers to "macho psychopaths" with extra Y chromosomes.
* The connection between criminal violence and an extra Y chromosome is touched upon by a character in ''Child of My Right Hand'', by Eric K. Goodman, as he thinks about genetic predispositions.
* In ''[[Literature/TheNeanderthalParallax Hybrids]]'', by Robert J. Sawyer, the link between an extra Y chromosome and a predisposition to violent behaviour is mentioned. A character even suspects another character of being XYY based on his physical appearance and personality.
* In ''Seduction'', by Catherine Gildiner, a character believes there are many XYY men in prison, whom she describes as violent and possessing low intelligence.
* In ''The Vision'', by Creator/DeanKoontz, Mary wonders whether it is possible for some people to be "born evil" and recalls having read about men with an extra Y chromosome, who are described as the "genetically ordained criminal type."
* At one point in ''Other Men’s Daughters'', by Richard Stern, several social issues and controversies are mentioned, including the "social peril of the XYY male with the criminal valence in his cells."
* In ''Transfiguration'', by Paul Weber, a doctor mentions this trope while discussing Apollonia's strange chromosomal abnormalities. The doctor states that people used to think XYY males had a higher rate of criminality but admits this has been shown to be false.
* This trope plays a role in ''Gone to Earth'', by Rick Boyer. Interestingly, the novel discusses not only the idea that XYY men have a propensity for crime and aggression but also mentions other features including having acne, prominent facial hair, lower intelligence and psychological issues as well as being taller and stronger.
* In ''The Labyrinth Key'', by Howard V. Hendrix, a character explains that society has always searched for a biological way of detecting future deviant behavior, and includes the XYY studies as an example.
* In ''Midnight Admirals'', by Douglas Muir, two characters discuss a killer and one explains the criminal can’t be XYY because he doesn’t display low intelligence.
* In ''Good News, Bad News'', by David Wolstencroft, a character refers to Latham as “XYY” and ponders how men were the cause of most wars and are blamed for almost all the violence in the world.
* In ''Soul'', by Tobsha Learner, a character searches for an explanation as to why some men are predisposed to violence and murder. She considers that an extra Y chromosome may be involved, but this explanation doesn't pan out.
* In ''Vickers'', by Mick Farren, a big muscular character who seems to like violence is referred to as "an Australian surfer with an extra Y chromosome."
* Interestingly, in ''Literature/{{Middlesex}}'', by Jeffrey Eugenides, XYY boys are referred to as "dreamers and loners," rather than the common stereotype.
* In "48 Hours," by Maarja Kangro, the main character recalls being taught in school that XYY men had a specific physique and a propensity for crime. She admits that this is no longer believed.
* In ''Zone of the Interior'', by Clancy Sigal, a mental patient with a background in science at one point claims to have proven that the cause of schizophrenia is the presence of an extra Y chromosome.
* In ''Series/TheRockfordFiles : The Green Bottle'', by Stuart M. Kaminsky, a character states she believes that Jamie, as well as his father, carry an extra Y chromosome, based on their behavior. Jamie Hanson is a violent criminal and his father is in prison for murder. This seems to be a common perception about Jamie, as another character also mentions the criminal may have an extra Y chromosome.
* At one point in ''Night Vision'', by Paul Levine, several characters discuss psychopathic killers. Pamela Maxson explains she studies both environmental and biological components, including possessing an extra Y chromosome, which might explain the behaviour of psychopaths. She claims that the XYY karyotype is four times more prevalent among murderers.
* This trope is prominently featured in ''Altered Biography: The Womb Years'', by Douglas Isaac, which includes a scene where one character suggests that a double Y chromosomal condition is partly responsible for the criminality of another character. The term "crime-o-somes" is also used in reference to this trope.
* In ''The Wizard of Seattle'', by Kay Hooper, Serena and Merlin speculate on why so many men in Atlantis look like Neanderthals and it is suggested that it could be due to having an extra Y chromosome. Merlin claims that many overly aggressive men are XYY and that the physical appearance of these brutish, Neanderthal-like men could also be linked to this abnormality.

* The Season 5 finale episode "Born to Kill" of ''Series/CSIMiami'' referenced this trope. A couple knew that their son had this condition and constantly treated him with suspicion because of it. [[spoiler:When their daughter accidentally kills their other daughter by pushing her down the stairs, she claimed ''he'' did it on purpose; this is what actually turned him violent.]]
* Played with in ''Series/CriminalMinds'', where a killer claims that he's XYY, and that's why he kills. However, Rossi replies that the study linking that condition to criminal behaviour was debunked years ago.
* An episode of ''Series/ForeverKnight'' centers on a legend that a (female) vampire who mated with an XYY male "higher than high, under the light of the full moon" would become human. The XYY human did have extra violent tendencies.
* ''[[ The XYY Man]]'', the first of Kenneth Royce's novels, was transferred to British television by Granada TV, in a three-part adaptation with Stephen Yardley playing Scott. The adventures of Scott caught the public imagination and ten more episodes followed in 1977.
* In the "By the Pricking of My Thumbs…" episode of ''{{Series/Doomwatch}}'', a teenager faces discrimination because he is XYY. The episode features a scientist who is convinced that having the extra Y chromosome predisposes a person to criminal behavior, but this position is meant to be seen as misguided.
* In "Born Bad", an episode of ''Series/LawAndOrder'', a lawyer argues that his client should not be found guilty since his extra Y chromosome predisposes him to be violent. [[spoiler: It [[GoneHorriblyRight went horribly right]], the client bought the argument so much that he begged to be jailed for life. He was a teen.]]
* In a 1993 episode of the TalkShow, ''The Phil Donahue Show'', a psychiatrist discusses a patient he believes committed murder due to having an extra Y chromosome.
* This trope is averted in one ''Series/MysteryScienceTheater3000'' episode, in which ''[[Series/RockyJonesSpaceRanger Crash of the Moons]]'' is riffed. Although a joke is made about a character in the movie having an extra Y chromosome, it is more a reference to having a learning disability than being aggressive.

* In a Nick Downes cartoon, a tough cowboy is known for "his fast gun, mean temper, and extra Y chromosome."

* In "Dipper vs. Manliness," an episode of ''Westernanimation/GravityFalls'', Dipper meets a race of [[ALoadOfBull Manotaurs]], who are meant to embody all aspects of manliness, including extreme aggression. When Dipper suggests, in a bit of reverse psychology, that they aren’t really manly, Testosteraur becomes enraged, claiming he has three Y chromosomes, pecs on his abs, and fists for nipples.

* The source of this idea was [[ some medical studies from the 1960s]], which claimed that the XYY genotype actually could cause a propensity for violent behaviour. The studies found that male prison inmates who were unusually tall had a slightly higher incidence of XYY than among the general population. Which was either a coincidence, or a false correlation that didn't properly account for a) how XYY men (like people with other chromosomal abnormalities) have an increased risk of learning disabilities that might hinder their ability to ''get away'' with a crime and b) selecting for height in itself increases the chance of finding XYY carriers. Though this idea has been discredited, it still pops up now and then.
* The "XYY criminal" theory was further popularised in the press when lawyers for the notorious serial rapist and mass murderer Richard Speck falsely suggested that he was XYY, in the hope that it would lead to a successful insanity defence.