Biological theories


Genetic theories:

Twin studies

• Compare monozygotic (MZ) twins with dizygotic (DZ) twins

• MZ twins share 100% DNA, DZ share 50%

• If the concordance rate (% of second twin that have the same as the first twin) is significantly higher in MZ twins than DZ, this indicates a genetic element

• If it is purely genetic, concordance rates would be 100% in MZ twins (so less indicates genes could be involved, but not solely genetic)


Twin studies - evidence

• Lange (1929)

• Mz twins showed a much higher degree of concordance than Dz for criminal behaviour

• 10/13 Mz twin pairs had both served time in prison, only 2/17 Dz twin pairs had

• Christiansen (1977)

• Looked at Mz and Dz twins in Denmark, one twin in each pairing had a criminal conviction

• The concordance rate for male Mz twins was 35% compared to only 13% for Dz twins

• For females the concordance rates were 21% for Mz and 8% for Dz

• This suggests that criminal behaviour could be influenced by genetics

• Raine (1993)

• Reviewed research on just delinquent behaviour of twins and found 52% concordance for MZ and only 21% for DZ.

• This suggests that genetics does play a part in determining criminal behaviours


Adoption studies

  • When a child is adopted away from their biological family

  • Look at % who share traits with biological parents despite being brought up away from them

  • Allows us to separate the influence of nature (genes we inherit) and nurture (the environment we are brought up in)

  • If the child is more similar to their biological parents, it suggests genetic influences. If they are more similar to their adoptive parents, it suggests environmental influences

  • This is most important when adoption takes place not long after birth (as otherwise there may be some environmental influences from their biological parents)


Adoption studies - evidence

  • Crowe (1972)

  • Found adopted children who had a biological mother with a criminal record had a 50% risk of having a criminal record by age 18 (compared to 5% chance in no criminal record)

  • This suggests that genetics plays a strong part in determining if someone will display criminal behaviour.

  • Mednick (1984)

  • Studied 13000 adoptees having at least one court conviction.

  • Convictions rates

  • 13.5% for biological or adoptive parent had no convictions

  • 20% one biological parent had a conviction

  • 24.5% both biological and adoptive parent had a conviction

  • This suggests that the difference could only be explained by genetics, as the biological parents had had little to no influence on the child's upbringing but the rates for these are Higher


Jacob’s XYY Study

XYY Syndrome

• Most people have 46 chromosomes (23 pairs)

• They carry all the genetic information we inherit from our parents (half our chromosomes from each)

• Two of these determine our biological sex

• Females are usually XX and males XY

• The Y chromosome makes a person male

• There are several chromosome abnormalities which involve an extra chromosome – one of these is known as ‘XYY’ when a male has an extra ‘Y’ chromosome


47,XYY "the criminal type"

• Adults with 47, XYY syndrome may exhibit a pockmarked face owing to severe acne during puberty.Growth is often accelerated during mid-childhood, so adults with this syndrome are often very tall. Severe acne may develop in puberty, leading to a pock-marked face in adult life.

• On the other hand, not all individuals are so affected by acne. Indeed, both physical abnormalities and behavioural aberrance’s may be absent.


XYY Syndrome - evidence

• It is suggested that males with an extra Y chromosome are “super males” and may be more aggressive and more likely to be violent than those with only one Y chromosome

• Jacob et al. (1965) suggested men with XYY are more aggressive than those with XY

• Price and Whatmore found XYY males to be immature and unstable with a strong tendency to commit motiveless property crimes

• Some studies show that XYY men are over-represented in the prison population

• 15 per thousand in prison

• 1 per thousand in the general population)


Physiological theories

Lombroso

• Features include

• Pre-social

• Unable to control impulses

• Reduced sensitivity to pain

• Like ‘savages’ or even apes – including long arms relative to lower limbs

• Large or forward projection of the jaw

• High cheekbones

• Flattened or upturned nose

• Low, slowing forehead

• Large ears

• Seen as ‘abnormal’ in modern society (but would have been normal in a more primitive society

Sheldon (1949)

• Advanced a theory that shares the principles of Lombroso in terms of physical form of a criminal

• Put forward three somatotypes (body types)

• Endomorph (fat and soft) – tend to be sociable and relaxed

• Ectomorph (thin and fragile) tend to be introverted and restrained

• Mesomorph (muscular and hard) tend to be more aggressive, adventurous and domineering