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Sex Chromosomes
Humans have 23 pairs of chromosomes, which contain the genetic information for various traits. One pair of these chromosomes, known as sex chromosomes, determines biological sex. Males typically have one X and one Y chromosome (XY), while females have two X chromosomes (XX). Each parent passes one chromosome from each pair to their child, so individuals inherit one X chromosome from their mother and either an X or Y chromosome from their father. The presence of the Y chromosome generally leads to male characteristics.
Chromosomal Abnormality and Crime
XYY syndrome is a rare genetic condition where males have an extra Y chromosome. This has led some researchers to suggest a link between the condition and increased aggression, as individuals with the syndrome may display more impulsive and violent behaviours. The theory proposes that the presence of an extra Y chromosome could predispose individuals to criminal behaviour, although later research has questioned the extent of this influence.
Jacobs' Research and Findings
Patricia Jacobs, a Scottish geneticist, studied the link between XYY syndrome and aggressive behaviour. Her research found that XYY men were overrepresented in prison populations, with 15 in 1000 male prisoners having the condition compared to just 1 in 1000 in the general public. Jacobs' findings suggested a possible connection between the extra Y chromosome and criminality, but she acknowledged that other factors may contribute to the higher rates of aggression seen in these individuals.
Characteristics of XYY Syndrome
XYY syndrome, also known as Jacob's syndrome, causes males to exhibit certain physical and behavioural traits, such as increased height, learning difficulties, impulsivity, hyperactivity, and, in some cases, aggression. While not all individuals with XYY syndrome display criminal or antisocial behaviours, the condition has often been associated with a greater likelihood of violent tendencies, although environmental factors are also crucial in shaping behaviour.
XYY Syndrome and the "Super-Male" Theory
The early theory of XYY syndrome linked the condition to the idea of the "super-male," suggesting that individuals with an extra Y chromosome were more aggressive, impulsive, and prone to violence. However, further research showed that not all XYY individuals exhibit these traits, and many lead non-violent lives. This highlighted that aggression cannot be explained by genetics alone, and that environmental and social factors also play a significant role in shaping criminal behaviour.
Case Study - John Wayne Gacy
John Wayne Gacy, a serial killer who murdered at least 33 men, was suspected of having XYY syndrome. His case was often cited as an example of the link between the syndrome and violent crime. However, no definitive evidence supported the claim that Gacy's actions were directly caused by his genetic condition. His violent behaviour was likely influenced by a combination of personal, psychological, and social factors, showing the complexity of criminal behaviour and the limitations of genetic explanations.
Court-Brown's Study (1965)
In 1965, Court-Brown conducted a study on 314 male patients with XYY syndrome. His research found that these individuals exhibited a higher likelihood of aggressive behaviour compared to the general population. Based on his findings, Court-Brown recommended that individuals with XYY syndrome be kept under medical supervision due to their increased propensity for violence. His study contributed to the early belief that there may be a genetic link between XYY syndrome and criminal behaviour, although this idea was later questioned by further research.
