Extra Y, Extra Violent

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:
 * A Y chromosome makes you male.
 * Therefore, an extra Y chromosome makes you more male.
 * Men are more aggressive and violent.
 * Therefore, an extra Y chromosome makes you even more aggressive and violent.

This trope is based on a couple of widely-reported studies from the 60's 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.

Film

 * Used in Alien. All the XYY males are automatically put on a penal colony.
 * Referenced in Scary Movie 3.
 * The biology research institute in The Cat O Nine Tails is researching it while the murders happen around it.

Literature

 * 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.

Live Action TV

 * 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.
 * 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.

Real Life

 * 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.