Are XYY men psychopathic killers?

There is no evidence that XYY leads directly to criminal behaviour or violence. That is good news because approximately 1 in 1000 men is born with an extra Y chromosome. Think about that – a small Canadian town of 2000 or a large urban high school should have one person with XYY. So be glad that there aren’t all these guaranteed violent people.

Men usually have one X and one Y chromosome but sometimes an extra Y chromosome gets into the mix. Unlike having extra copies of the autosomes (non-sex chromosomes), the extra Y doesn’t seem to have any major effects. These men generally grow up without knowing they have an extra chromosome. If you do know they have an extra Y chromosome, you may notice that they are slightly taller (on average 7 cm more), have more acne in adolescence and some men do have learning difficulties with IQs 10 to 15 points lower than their siblings. But you often have to look for these subtle things.

But the image of the XYY violent criminal is a common misconception – why? The first XYY patient was slightly aggressive so it was asked whether XYY could explain aggressive behaviour in men. In 1965, a British geneticist examined the chromosomes of 197 men in an institution for dangerous criminals and found 8 XYY men. This seemed very high so it was concluded that XYY men become criminals but the proper controls to this and other studies weren’t done. After the initial study, many follow-up screenings for XYY men were done in penal or mental institutions and confirmed the XYY and violence connection. But one large problem with these studies was ascertainment bias – if you look for something, it is often there. Because these men were already judged to be ‘abnormal’ by society, finding XYY chromosomes seemed like an easy explanation for behaviour difficulties. But sometimes things are just a coincidence.

Children with sex chromosome abnormalities can be picked up prenatally or postnatally. If they are diagnosed prenatally, it is usually because the mother has undergone routine prenatal diagnosis. Almost all (>90%) of the children are normal. But the postnatal cases are being brought to the clinic because someone is concerned. It is tempting to say that the problems in the child (postnatal case) is due to the sex chromosome abnormality – but then you would expect the same number of problems in the prenatal cases and that isn’t seen. So maybe there is a different cause of behavioural difficulties in some of these XYY children.

Again, why has XYY and violence pervaded our consciousness? Well, it seemed plausible – men seem more aggressive than women and the Y makes a man during development so having two copies would make you twice as aggressive. Plus, there was a great slogan of the ‘criminal chromosome’ in the press. Why was it in the press? There was a famous murder trial, Richard Speck. He had brutally raped and murdered 8 nurses in Chicago in 1966 and then at trial he claimed that he was XYY and couldn’t help his violent tendencies. The jury still found him guilty. Interestingly, a repeat karyotype was done a few years later and he was found to have a normal 46 XY karyotype so it definitely could not have explained his murderous ways. Yet his case helped bring this idea into the mainstream. And it still persists. It is has been raised as a defense on the Law and Order shows. The movie Aliens 3 is set in a penal colony for XYY males that are thieves, murderers, and rapists with no hope of rehabilitation due to their genetic make-up. And the idea that XYY is a cause of violence, aggression and criminal behaviour did make it into textbooks so if they have not been properly edited, the misinformation may still be there.

This is a dangerous misconception because no direct link has been made between aggression or criminal behaviour and an extra Y chromosome. But because people have this belief, there is a stigma to boys with this chromosome complement. If you knew you had XYY, would you worry? If you were pregnant with an XYY baby, would you worry? There is no evidence that you should but there is such a thing as a self-fulfilling prophecy and if enough people tell you that you will become a violent criminal…

March 2009