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
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