I've been determined to write about XYY males for a month or more but have been distracted by more pressing issues and also by the didactic necessity to explain the anomalies (previously I, II) which science has discovered in the count of human chromosomes. In the late 1950s and early 1960s, cytogenetics [the microscopic study of the details of chromosomes] was king. Barabara McClintock was tracking microscopic knobs on maize chromosomes associated with detectable attributes of the whole plant. Mary Lyon was telling the world about the inactivation of one of the pair X chromosomes. And that's just some of the women who contributed to the chromosomatization of genetics.
The Y chromosome [59.4 million basepairs Mbp] is not the shortest chromosome in our complement: 19 [59.1 Mbp] 21 [48.1 Mbp] and 22 [51.3 Mbp] are all shorter. But it hosts far and away few protein coding genes - only 45 of them at the latest best-guess count. Shortie Chr 21 has 5x more genes and the others many more; up to Chr1 with more than 2000 known genes. For us blokes, the most important gene of these 45 is SRY coding for a protein called TDF testis-determining factor which triggers the development of the testes from a primordial blob of cells that could go either way - testis or ovary. What happens if by genetic accident you inherit two Y chromosomes from your father? Would it matter? You'd presumably switch the primordial gonads into a pair of testes and you'd acquire all the advantages that men have in our tilted society.
The first case of XYY was published in the Lancet in August 1961 and it was revealed entirely by accident when trying to count the chromosomes of a guy [he was American] who had fathered two daughters with Down Syndrome. That must have been a careful piece of research because it would have been all to easy to mistake the extra teeny Y chromosome as an extra tiny 21 and bingo! 2 DS daughters was explained. While the report was in press, on the other side of the world in Melbourne, a hood called Robert Tait was robbing a house. When the owner of the property disturbed him, he bludgeoned her to death and committed a number of other acts that had the tabloids and their readers "fascinated". Tait was sentenced to death; his lawyer pleaded insanity; plea was rejected; date of hanging set; before finally the sentence was commuted to life imprisonment. At the time, the Australians and citizens from many other civilised countries were losing the taste for hangings. Six years later, during a cytogenetic survey of Australian prisons, Tait was revealed to have an extra Y chromosome. As were a succession of violent criminals in Australia, Germany, the USA. In none of these early cases was an XYY karyotype used by the defense but as early as 1961 and Dr Court Brown was suggesting that like the plea for insanity, which had been enshrined as the M'Naghten Rules, XYY should in itself provide extenuating circumstances for criminal behaviour. I'll also assert that none of these early blood-sampling surveys among prisoners would pass modern standards of informed consent.
The theoretical thought becoming the data-collection exercise deed, Brown's colleague Patricia Jacobs carried out a chromosome survey of men banged up in the Carstairs maxiumum security hospital outside Glasgow. She found the rate of XYY was about 3% in a trawl through 200 inmates which is a statistically significant difference from the prevalence in the general population which is about 0.15% or 20x less. The XYY men from Carstairs were significantly taller than their XY fellow inmates. If you want a sample of XYY men and have limited resources, you might be advised to go to Broadmoor Hospital and pick the tallest men in that Institute. Early studies suggested that men with acne were also more likely to be XYY but that was soon dismissed as ascertainment bias within a tiny sample. XYY, acne and mass-murder were clagged together with the epithet supermale by a biochemist called Mary Telfer and this 'syndrome' was exhaustively reported in the New York Times in 1968. Abnormal ECG, EEG, leg ulcers, genital abnormalities have all been claimed to be more common among XYY men, but the statistical evidence for any of these associations is really no better than for acne. One rather interesting finding is that, when compared to others in the same max security institution, XYY men are taller but less violent. It is suggested that having done something bad the XYY men, by looming over the judge and jury, put the frighteners on them and the tall chaps get convicted by irrational fear.
You can see why the XYY story has acquired (long) legs. In our cartoonish society, men are aggressive and women are meek, men and taller too . . . and they have a Y chromosome. The NYT series on XYY has embedded itself in our compendium of truthiness and has been reprised in Kenneth Royce's book The XYY Man and numerous lazy-arsed scripts in films and on television. There are far too many false positives to allow us to damn all men who have this extra scrap of DNA: more than 97% of the 30,000 XYY men in Ireland (and 2 million in the USA!) are decent law-abiding taller-than-average blokes. Before you get too judgmental and certain, you might re-read Richard Gill's demolition of the case against "criminal" Lucia de Berk. Or the righteous wrongness of witch-hunting children who aren't the spit of their parents.