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Extra Y, Extra Violent

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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 towards 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:

This trope is based on a couple of widely reported studies from the 1960s which didn't pan out on further research. They claimed that the XYY genotype 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 shows up now and then.

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. See also Villainous Lineage, another trope about a supposedly genetic basis for evil/criminality.


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    Anime & Manga 
  • Yujiro Hanma from Baki the Grappler very likely has one, given his characteristics and possoble role in the story.

    Comic Books 
  • 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.
  • The story "X-tra X" in Creepy Magazine #34 has a strange inverted example — here, violent behavior is linked to an extra X chromosome (known as Klinefelter syndrome), hence the title. To make matters more complex, the story focuses on a genetic mutant whose extra X chromosome is sensitive to the gravity of the full moon, producing a fluid that induces a physical transformation along with a violent psychological state — in short, making the man a werewolf.
  • Team 7: 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.

    Fan Fiction 
  • In Babel Fish, an 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.
  • In Fourth Down, a Fire Emblem fanfic, a new football coach wonders why Ephraim is so aggressive. Eirika suggests it may be an extra Y chromosome.
  • The Ultimate Demon, a Charmed (1998) fanfic, features a character who 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 A Very Traditional Conversation, a YuYu Hakusho fanfic, a character discusses the link between XYY and aggressive behavior, although they explain that the violent nature involved can be controlled with effort.
  • At one point in the Dark Angel fanfic When the past comes ahaunting, a character is skeptical that a genetic abnormality could cause criminal behavior in an otherwise stable character, stating that only an extra Y chromosome could lead to violent behavior, but this would make them "a serial killer and quite asocial."

  • Alien³: 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.
  • The Terzi Medical Institute in The Cat o' Nine Tails is researching the supposed link between XYY chromosomes and criminality, while the murders happen around it. Everyone at the Institute has submitted blood samples to be tested for the research. The killer is one of the scientists, who tested positive for XYY chromosomes, revealing him as a sociopath. He first breaks into the Institute to hide this evidence, then murders a blackmailer, then keeps on killing to cover up his involvement in the ever-longer crime spree...

  • 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.
  • 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 Sarah Ott's creative writing piece "Charles", this is one of the misdiagnoses the author gives to her classmates. The classmate is actually just moody.
  • In Child Of My Right Hand by Eric K. Goodman, the connection between criminal violence and an extra Y chromosome is touched upon by a character as he thinks about genetic predispositions.
  • 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.
  • 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 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.
  • The short poem "It's Easy As XYY" refers to this trope, linking an extra Y chromosome to violence.
  • One character in I Will Fear No Evil is said to be XYY, which is linked to their murderous actions. Interestingly, later printings changed this reference to "XYZ" rather than "XYY".
  • 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.
  • Interestingly, in Middlesex, XYY boys are referred to as "dreamers and loners" rather than the common stereotype.
  • In Midnight Admirals by Douglas Muir, two characters discuss a killer, and one explains that the criminal can't be XYY because he doesn't display low intelligence.
  • 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 Neanderthal Parallax: In Hybrids, 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.
  • 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.
  • At one point in Other Mens 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 The Rockford Files: 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 that the criminal may have an extra Y chromosome.
  • The Chromos in "Salvation through Speed" are men bred with an extra Y chromosome to increase their aggression. The idea was that this would make them better military grunts and security personnel. In reality, the extra Y chromosome did not have the desired effect, but the name "Chromo" continued to be used.
  • In the short story "Sam, Soren and Ed" 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 defuse the situation.
  • 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 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.
  • In Soul, 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 the Star Trek: The New Voyages 2 short story "The Procrustean Petard", the Enterprise crew is gender-flipped by an alien device which doesn't know how to deal with Half-Human Hybrid Mr. Spock, so it makes him XYY instead, making him see his two now female friends as weak and useless.
  • In The Stranger In Goldrush by Sheila Bush, a character briefly refers to "macho psychopaths" with extra Y chromosomes.
  • Mentioned in Succulent Prey by Wrath James White when a character states that the link between an extra Y chromosome and serial killers has been shown to be false.
  • 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.
  • In Truth Will Out by A.D. Garrett, a character discusses the XYY/violence myth at length, including how it led to a wrongful conviction.
  • 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".
  • In The Vision by Dean Koontz, 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".
  • 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 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.
  • In the short story "Yesterday's Leeches" by Lee Chiao, the main character, whose sexuality is repeatedly questioned, at one point tries to convince himself that he could be XYY and a super-male. While he has psychotic and aggressive tendencies, this chromosomal state is not confirmed.
  • 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.

    Live-Action TV 
  • Played with in Criminal Minds when 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.
  • The CSI: Miami Season 5 finale episode "Born to Kill" references this trope. A couple knew that their son had this condition and constantly treated him with suspicion because of it. 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.
  • In the Doomwatch episode "By the Pricking of My Thumbs...", 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.
  • An episode of Forever Knight centers on a legend that a (female) vampire who mates with an XYY male "higher than high, under the light of the full moon" would become human. The XYY human does have extra violent tendencies.
  • In the Law & Order episode "Born Bad", a lawyer argues that her client should not be found guilty since his extra Y chromosome predisposes him to be violent. It goes horribly right; the client buys the argument so much that he begs to be jailed for life. It should be noted that the police and prosecution don't accept that theory, and a witness explicitly says that it's discredited.
  • This trope is averted in the Mystery Science Theater 3000 episode "Crash of the Moons". 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 1993 episode of the Talk Show The Phil Donahue Show, a psychiatrist discusses a patient who he believes committed murder due to having an extra Y chromosome.
  • 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. This is a slight subversion, as Scott is more of an amiable cat-burglar than any sort of violent person. In fact, Scott is probably more of a Lovable Coward or a Raffles type.

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

    Western Animation 
  • Gravity Falls: In "Dipper vs. Manliness", Dipper meets a race of 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.

Alternative Title(s): XYY, Extra Y Chromosome Extra Violent