Human cell genetics is a relatively new profession. It wasn't until the 1950s that reliable techniques were developed to visualise the morphology and establish the number of chromosomes in the human genome. What followed was a period of rapid advancement during which the major chromosome aneuploidies were described. One of these conditions, Jacobs syndrome, was characterised in 1959. In humans, the presence of the Y chromosome determines maleness. The 'normal male' is bestowed with a single, meagre Y. In truth, the Y chromosome is a stubby nondescript chromosome with very few expressed genes The ever present SRY gene (testes determining factor) is responsible for driving the embryo down the male developmental pathway. In addition, there are genes determining stature and a gene for hairy ears- the bane of every man over 50.
Men with Jacobs syndrome are blessed with not one, but two, Y chromosomes. Unlike most chromosome disorders, men with an extra Y are clinically unremarkable. Most XYY men remain undiagnosed and lie below the clinical radar. They don't suffer major organ defects or look odd. They do have a tendency to tallness due to a double dose of stature genes and their average IQ, as a collective group, is lower (10 to 15 points) than the general male population. For the most part, the IQs of XYY men lie within the normal or low normal range, whilst some may exhibit a degree of mild mental deficit.
When Jacobs syndrome was first discovered, the presence of the extra chromosome gave rise to the unfortunate designation of 'Super male'. If one Y is required for maleness, then surely two would result in a highly masculinised male. This view became reinforced when it was discovered that XYY males were found to be enriched in the prison system. In the general population, the incidence of the condition is about one in a thousand men. However, when we observe the prison population the incidence rises to one in a hundred. Early speculation centred on the possibility that XYY men were highly aggressive and therefore more likely to be involved in violent crime. However, when further investigated it became apparent that the type of crimes committed by these men was no different from the majority of inmates- low-level crime, mostly theft and damage to property Thus it seems that XYY inmates are not 'Super' in any respect, just petty criminals.
The association of the condition with criminality became enshrined in the British consciousness due to a popular television series of the 70s, called 'XYY Man'. The underlying premise throughout the series centres on the main character who is driven to crime as a consequence of his extra Y chromosome. This raises an interesting side issue not pursued by the series: To wit, if the man has no choice in his actions due to innate programming is he considered morally culpable for his wrong doing? If it can be determined that he is unable to restrain his criminal activity because of the extra chromosome, would it be right to punish him for his transgressions? This highlights an interesting ethical conundrum not easily resolved. Sadly, I don't have space in this article to consider the dilemma, here. Digression endeth.
So why is it we see a greater incarceration rate amongst these men? The ultimate answer is rather mundane. When we sample the IQ amongst prison inmates it tends toward 10 to 15 points lower than the general population. Should we be surprised, that stupid folk make stupid decisions in their life? Tis a simple matter of IQ demographics. Ain't dat the sad truth?
It would be wrong to label XYY men as criminals or potential criminals, unless incarcerated. The majority lead uneventful lives and remain, unrecognised by the public, doctors, and the judiciary system. That aside, if one of these 'unremarkable males' appeared in my home, brandishing a pointed stick, or even a bunch of sundry fruit, I would gun him down like a dog and move the corpse to my son in law's pig farm where the unclean and besmirched corpse would become an integral part of the porcine faecal environment. Nuff said.