Genetic theories of Criminality

Question 1

The three genetic theories of criminality are…

Answer 1

XYY theory (Jacob et al)
Adoption studies (Mednick)
Twin studies (Lange; Christiansen)

Question 2

What genetic abnormality is the XYY theory interested in?

Answer 2

XYY theory looks at men with an extra ‘Y’ chromosome. A man normally has one X chromosome and one Y chromosome. XYY men have one X and two Y chromosomes. Those with the XYY genetic abnormality are often described as ‘super male’.

Question 3

Men with XYY were found to be…

Answer 3

more aggressive than normal ‘XY’ men, according to Jacob et a. (1965)

Question 4

What is the prevalence of XYY in society?

Answer 4

Studies suggest that there are 1 in 1000 men with XYY in society

Question 5

What proportion of prisoners have XYY?

Answer 5

Studies suggest that there are 15 in 1000 prisoners with XYY.

Question 6

Who was the main theorist behind the XYY theory and what year was the study carried out?

Answer 6

The key theorist behind the XYY theory was Jacob - Jacob et al (1965)

Question 7

What did the Jacob et al (1965) study find?

Answer 7

Jacob’s study was carried out at a maximum security hospital in Scotland. It found that 9 out of 315 patients had an extra Y chromosome.
Further studies found 15 sufferers in every 1,000 in prison, compared to just 1 sufferer in 1,000 in the general population.

Question 8

Give an example of an offender that had XYY

Answer 8

Criminologists suggest that the serial killer, John Wayne Gacy had XYY syndrome.
He sexually assaulted, tortured and killed at least 33 men in the USA.

Question 9

Give two strengths of XYY theory

Answer 9

• 15 in 1000 prisoners is a substantial figure (well above the 1 in 1000 in the general population figure), providing credibility to the theory.
• Studies by Adler et al. (2007) indicated that it is possible that aggressive and violent behaviour is at least partly determined by genetic factors.

Question 10

Give three weaknesses of XYY theory.

Answer 10

• Studies have shown that genetic abnormalities are widespread in the general population, and therefore don’t explain aggression.
• Focusing too heavily on genetics ignores the behaviourist approach.
• Theilgaard (1984) compared XYY men to XY and found that the characteristic of aggression was not associated with the XYY men.

Question 11

Who were the main theorists behind the Twin studies and what years were their studies carried out?

Answer 11

Lange (1929) and Christiansen (1977)

Question 12

What is the difference between MZ twins and DZ twins?

Answer 12

Monozygotic (MZ) twins are identical and share an egg, and 100% of their DNA.
Dizygotic (DZ) twins come from different eggs and only share 50% of their DNA.

Question 13

What are concordance rates?

Answer 13

Concordance rates are the probability (as a percentage), that if one twin has a characteristic the other twin will have the characteristic too.

Question 14

What did Johannes Lange’s study (1929) find?

Answer 14

Lange found that MZ twins showed a much higher degree of concordance than DZ twins for criminal behaviour.
He investigated 13 MZ twins and 17 DZ twins.
10 out of 13 MZ twins had both served time in prison.
Only 2 out of 17 DZ twins had both served time in prison

Question 15

What did Christiansen’s study (1977) find?

Answer 15

Christiansen investigated 3,586 twin pairs born in Denmark between 1881 and 1910. He found that male twins were more likely to share criminal characteristics compared to female twins.
Males:
MZ concordance rate = 35%
DZ concordance rate = 13%
Females:
MZ concordance rate = 21%
DZ concordance rate = 8%

Question 16

Give three strengths of Twin Studies.

Answer 16

• The biological relationships occur naturally, so these are natural experiments.
• Christiansen (1977) supports the view that criminality does have a genetic component.
• The results of twin studies have helped in prevention of vulnerable disorders.

Question 17

Give three weaknesses of Twin Studies.

Answer 17

• Early twin studies (such as Lange, 1929), didn’t have proper controls, and lacked validity as to whether the twins were DZ or MZ, as they only based this on appearance, not DNA.
• The small sample size involved in twin studies may not be representative of the general population.
• If twins are brought up in the same environment, criminality could just as easily be related to nurture as to genetics.

Question 18

Who were the main theorists behind the Adoption Studies and what years were their studies carried out?

Answer 18

Hutchings and Mednick (1975)
Mednick et al. (1994)

Question 19

What do adoption studies suggest about criminal behaviour?

Answer 19

Adoption studies suggest that criminal behaviour can be explained through comparisons between biological and adoptive parents. If, in criminal behaviour, the child is more similar to their biological parents than their adoptive parents, it may suggest a genetic basis for criminality. If the child is more similar to their adoptive parents than their biological parents, there would be a stronger environmental argument for criminality.

Question 20

What did the study by Hutchings and Mednick (1975) find?

Answer 20

Hutchings & Mednick (1975) studied 14,000 adopted children and found a high proportion of boys with criminal convictions had biological parents with criminal convictions too. This suggests there is a strong link between genetics and criminality.

Question 21

What did the study by Mednick et al. (1994) find?

Answer 21

Mednick et al. (1994) found no relationship between the number of criminal convictions of adoptive parents and their adopted children, but did find a significant correlation between the number of criminal convictions of the biological parents and their offspring.

Question 22

State two strengths of Adoption Studies.

Answer 22

• As adopted children are exposed to a different environment to their biological family, it is easier to separate genetic and environmental factors.
• Studies have concluded that there is a correlation between adopted children and their biological parents.

Question 23

State three weaknesses of Adoption studies.

Answer 23

• The age of adoption may mean the adopted children have already been influenced by either their natural parents or their foster environment.
• Information about a biological family is not always available.
• The adoption process is not always random, as often children are placed with parents similar to their biological families.