XYY

• Chromosomes

  Pairs of genetic material that contain the instructions for the development and functioning of living organisms
• Humans have 23 pairs of chromosomes
• Conception

  1. Embryo receives 23 chromosomes from mother's egg

  2. Embryo receives 23 chromosomes from father's sperm
• Pairs 1 to 22

  • Identical or nearly identical
• Pair 23


  • Sex chromosomes, either X or Y
• XX pair

  Female
• XY pair

  Male
• XYY syndrome

  Genetic condition where a male has an extra male (Y) chromosome within the 23rd pair of chromosomes instead of the usual two (XY)
• XYY syndrome results in a total of 47 chromosomes
• XYY syndrome occurs in 1 in 1,000 male births
• XYY syndrome is not an inherited condition but occurs randomly at the time of conception
• Many men with XYY don't even know they have an extra chromosome
• XYY syndrome

  • Most boys will have normal development
  • Some may grow faster and taller than usual
  • Some may have a slightly lower level of intelligence, although still within the normal range
  • Some may have behaviour problems and be easily distracted
• 'Super-male' syndrome

  Previous term used to describe XYY syndrome, implying it makes men overly aggressive and lack empathy
• Early studies reported a connection between XYY, aggression and crime
• Jacobs et al (1965) study


  • Chromosome survey of male patients at a state hospital in Scotland
  • Found men with XYY chromosomal pattern were over-represented in prison populations (around 15 per 10,000) compared to the general population
• XYY Syndrome physical and psychological symptoms

  • Taller than average
  • Physically active
  • Behavioural problems such as impulsivity, being distracted easily
  • Lower intelligence compared to their siblings
  • Severe acne
  • Delayed motor and language development
• A weakness of the XYY syndrome explanation of crime is the lack of evidence
• A meta-analysis by Re & Birkhoff (2015) concluded that there is no link between XYY and offending behaviour
• Prevalence of XYY males within prison populations

  Might be better explained by social factors such as the fact that XYY characteristics make it harder for such men to integrate into society and find work
• The XYY syndrome theory is not a credible explanation of crime
• XYY only affects biological males so findings cannot be routinely applied to females
• Research into gender differences in amygdala function has produced conflicting results with several studies reporting no difference in structure or function
• The implications of amygdala research for sex differences in criminal activity have not been established
• Biological research may lead to new ways to assess criminal culpability

  Neural injury should be viewed in the same way as mental health is in court, reduced as the individual has less responsibility for their crime
• Biological explanations of criminal behaviour are reductionist
• Biological explanations ignore the fact that offending behaviour is more likely to occur in context of social deprivation, poor education and dysfunctional role models</b>
• Biological explanations may distract us from a truer understanding of crime
• There is increasing evidence that offending can, at least partly be influenced by biological factors, particularly in relation to brain structure and function
• Research supporting the effects of brain injury and the amygdala on crime is compelling, and may prompt society and the legal system to have to rethink notions of criminal responsibility and 'guilt' in the future
• Offending behaviour is a complex social activity which may not be adequately explained by biological factors alone