Since 1905, when Nettie Stevens and E.B. Wilson first identified the XY sex-determination system found in humans and many other species, it has been recognized that males and females differ in their sex chromosome arrangement. While males are heterogamic with two distinctive sex chromosomes (XY), females are homogamic with two of the same type of chromosome (XX). In approximately one out of 160 live births however, chromosomal anomalies occur with children being born with an abnormal number of chromosomes (aneuploidy). Among the more well-known sex-chromosomal disorders including Turner Syndrome (single X chromosome), Klinefelter's Syndrome (XXY syndrome), and Triple-X Syndrome (XXX chromosome karyotype).
When the 47,XYY karyotype was first identified in 1959, geneticists questioned whether it could be considered a disorder since there were no clear indications of pathology. Most XYY males develop normally and have no idea that their genetic structure is abnormal. Although research into XYY males has shown no significant difference in testosterone levels, differences have been found on other physical characteristics including physical height, impulsivity, short-temperedness, and enuresis. XYY males are also more prone to certain types of learning difficulties as well as delayed speech and language skills. Although XYY males have not been found to be more aggressive overall, early media reports (helped by a 1965 research study by Patricia Jacobs examining XYY males in institutional settings) declared that the XYY karyotype was the hallmark of "supermales" who were prone to criminal violence. The term criminal syndrome became briefly popular with some reports claiming that XYY males were 25 % more likely to be incarcerated than males with a standard karyotype.
As the mass media began publicizing existing XYY research (some stories referred to the Y chromosome as the "murder chromosome"), the legal community quickly took interest. If the XYY syndrome was linked to violent behaviour, wouldn't it be a valid defense in criminal trials? Which brings us to the Richard Speck case.
When Richard Speck brutally tortured and murdered eight student nurses in the Chicago area during the night and early morning of July 13 and 14, 1967, the question of his mental competence to stand trial became paramount. As one of the most notorious mass murderers in Illinois history, the possibility that Speck has an organic brain syndrome that reduced his inhibitions and led to his murderous actions was addressed by an impartial panel of five psychiatrists and one surgeon (selected by defense and prosecution respectively). During this assessment phase, Speck's public defender, Gerald Getty, decided to try a new angle. Since Speck had many of the characteristics associated with XYY syndrome (physical tallness, mild mental retardation, and acne), couldn't his karyotype have predisposed him to violence? Unfortunately for Speck, the genetic profiling requested by Getty showed the normal 46,XY karyotype.
In January 1968, Gerald Getty contacted biochemist Mary Telfer about her research into XYY incarcerated males. Given her published work with XYY males in jails and prison systems in Pennsylvania, she seemed the ideal expert to explore the question of Speck's mental status. Based on Telfer's own observation that most of the XYY males she dealt with had facial acne she made the erroneous conclusion that Richard Speck was an XYY male as well. Since Speck had virtually all of the characteristics associated with the 47,XYY karyotype that Telfer had found to date (borderline mental retardation, history of impulsive violence, physical tallness), Mary Telfer made a number of statements to the popular media reiterating Richard Speck as being an archetypal XYY male. The New York Times, Newsweek, and Time magazine, among other news sources, helped reinforce the perceived link between XYY syndrome and violence. It probably didn't help that his defense attorney attempted to argue that Richard Speck was prone to uncontrollable urges and, as such, could not be fully responsible for his actions. This novel defense was quickly dropped when the geneticist who had initially determined that Speck was XY went public to refute the existing claims. Richard Speck was sentenced to death which was later commuted to life imprisonment (he died in 1991).
Despite more recent high-profile cases involving XYY males convicted of shocking crimes (Arthur Shawcross and John Wayne Gacy are two examples), research into the prevalence of XYY males in forensic populations has not yielded consistent evidence of a clear role for the karyotype in violent behaviour. Given that most XYY males in the general population never commit violent crimes (and, in fact, often remain unaware of their chromosomal abnormality), critics of research focusing on incarcerated males point out the obvious selection bias at work. These same critics also argued that XYY research led to potential stigmatizing of XYY males. While the controversy rages on, researchers continue to report significant findings for XYY males in terms of increased mortality, physical height, neurocognitive deficits and autism. Although behavioural differences are still being reported (including incidence of sexual violence), no clear consensus on the XYY-violence link seems likely to emerge at this point.
Despite the issue of whether XYY males are prone to criminal behaviour remaining unresolved, it does represent a good example of the potential dangers involved in misrepresenting behavioural genetics research in the popular media. Lingering rumours of Richard Speck being XYY still surface at times and XYY males in prison continue to risk being stigmatized as being potentially violent, regardless of their actual offending. Although the field of behavioural genetics is still in its infancy, more questions surrounding how genetics affect behaviour will certainly emerge in future.