Men with an extra Y chromosome is a pretty common genetic disorder (1 out of 1,000 biologically male humans) that, at its worst, has only two symptoms: above-average height and an increased susceptibility to 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 (see the Real Life
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.
- 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.
- Referenced in Scary Movie 3.
- The biology research institute in The Cat o' Nine Tails 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 Star Trek 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.
- 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 Season 5 finale episode "Born to Kill" of CSI: Miami referenced 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.
- Played with in Criminal Minds, 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 Forever Knight 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 one episode of Doomwatch, “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.
- In "Born Bad", an episode of Law & Order, a lawyer argues that his client should not be found guilty since his extra Y chromosome predisposes him to be violent. It went horribly right, the client bought the arguement so much that he begged to be jailed for life. He was a teen.
- Serial killer Richard Speck was widely (and incorrectly) reported to be XYY, which popularized this trope.
- 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.