XYY-Death of the Übermensch

Myth Debunked-

Some of us might remember our biology lessons better than others, but there are some facts that we all know for sure, such as plants get the energy from sunlight, that things spread from areas of higher concentration to areas that are lower, and that when it comes to genetics, if you have two X chromosomes you are biologically female, but if you inherit a Y chromosome from your father (XY) the resulting baby develops as biologically male, right?  That is what we all were taught, however it’s really not that simple. It turns out that in 1961, a scientist called Dr. Avery Sandberg was discovered to have an extra Y chromosome (XYY)(1), and shortly after that a team of scientists went to study this phenomena in a place where you find “willing” participants without all that worry of “ethical consideration”, the prison system.  British cytogeneticist Patricia Jacobs and her team conducted a series of chromosomal studies on the 315 patients at Edinburgh’s State Hospital, a high security ward for dangerous criminals (2). From these surveys, Jacob observes that a high proportion of patients with XYY chromosomes exhibited similar physical and behaviours characteristics; including above average height, a degree of mental retardation and particularly violent criminal records. From this point, the idea of these “hyper males” was created; men with the power of two Y chromosomes, exhibiting more violent tendencies than any “normal” man. The fact that Jacob’s team were conducting their observations in a prison for dangerous criminals did not seem to overshadow this revelation. A later study was conducted in 1968 in the USA by Telfer et al (3), that conducted further chromosomal surveys on males with developmental issues, and found five boys with XYY chromosomes who were tall with relatively long limbs, with acne scars, some degree of mental retardation and aggressive behaviour, seemingly providing further “evidence” for Jacob’s theory about the dangers of XYY individuals. 

18 Jul 1966, Dallas, Texas, USA – Richard Speck
-Google commons

Around the same time as these studies, the news in the USA had focused its attention on a brutal series of murders and rapes committed by Richard Speck, a tall man with visible acne scars. Many suspected Speck of being one of these violent “supermales”, with The New York Times printing a set of articles reporting on these scientific findings and the danger that XYY individuals pose, and even though later testing showed that Speck did not possess an additional Y chromosome, it was too late. By this point the zeitgeist had its claws firmly in the idea of these dangerous super males, with little acknowledgment that these ideas came from skewed observations and unfortunately only slowly fell out of favour over the past few decades.

 As it turns out XYY males are much more common in society than you might think, 1/1000 boys in fact (4), and most are just like Sandberg, and exhibit no observable symptoms, and no predilection towards violence.  So why did this idea proliferate? Most likely because if confirmed some outdated ideas about gender roles, and it fitted within a lazy generalisation. During the same study, Jacobs’ team also observed a number of patients with XXY chromosomes, but they did not look any deeper to see if any of these patients expressed any stereotypical “female” characteristics; it did not fit the narrative of the violent criminals they were studying.

The science of sex and gender is actually really complicated, and how people want to present themselves in this world is a highly personal and introspective experience. However, for the longest time, we as a society decided that it is important to be able to separate each other into one of two factions, male or female. The discovery of the sex chromosomes seemed to perfectly encapsulate this binary thought process for a long time. But it’s really not that simple. Yes, while  if a person has XX or XY chromosomes provides a somewhat adequate rule of thumb, it is not the whole story and there are actually many, many more possible combinations of these chromosomes which result in a healthy baby, whose gender identity may present somewhere across a spectrum,  as well as conditions which may ignore the signals from these chromosomes anyway.

Possible karyotypes for male and female phenotypes for, taken from TED talk- The weird history of the “sex chromosomes”

When it comes to the other chromosomes, most of them are variations of each other, but that’s not the case for X and Y.  Whereas X is completely formed, Y is left lacking. After the discovery of these chromosomes in 1905, scientists have debated what to call this pair, with them ending up begrudgingly settling on the “sex chromosomes”, but they are far more than just there for determining the biological sex (well X at least). Without the X chromosome a developing foetus will not make it to term as it does a lot of important stuff, and only 4% of the genetic material in the X chromosome has anything to do with sexual development. It turns out that there is a lot more to sex and gender than just the presence of a Y chromosome, DNA variations throughout the genome play their part, as well as external factors. If there is any question of gender in an adult, doctors do not use the same chromosomal benchmarks as they would for developing babies, there are too many other factors to consider. But perhaps in the near future we may start to be more accepting of the fact that life is rarely binary, and such gender tests may at some point seem far less important.  


  1. Sandberg, A.A., Koepf, G.F., Ishihara, T. and Hauschka, T.S., 1961. An XYY human male. The Lancet278(7200), pp.488-489.
  2. Jacobs, P.A., Brunton, M., Melville, M.M., Brittain, R.P. and McClemont, W.F., 1965. Aggressive behaviour, mental sub-normality and the XYY male. Nature208(5017), pp.1351-1352.
  3. Telfer, M.A., 1968. Diagnosis of gross chromosomal errors in institutional populations. Pennsylvania Psychiatric Quarterly.
  4. https://rarediseases.org/rare-diseases/xyy-syndrome/
  5. https://www.ted.com/talks/molly_webster_the_weird_history_of_the_sex_chromosomes

Further resources :


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