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2. Chromosome and abnormalities
Blood samples can be taken to look at the chromosomes to see if they have any abnormalities
that might cause criminal behaviour.
Normally Males have an XY Chromosome pattern.
But in the 1960 researchers discovered with an extra Y chromosome (XYY) are more likely to be
violent and Criminal.
This condition is not inherited so does not run in the families. Even through it may be a genetic
reason for criminality.
It is a very rare disorder but some men who have this disorder have been found to be more
aggressive and slower at learning than average.

3. Murderers
The following murderers were found to have the chromosome abnormality XYY.
Richard Speck (Murdered 8 student Nurses)
John Wayne Gacy The Clown Killer(Murdered 33 boys and men)
Aurthur Shawcross (murdered 11 Women)

4. Starter questions:
1. Describe two ways of studying whether criminality is biological.
2. What problems might there be with saying criminality is
biological?
3. Minority report – Discuss!

5. Theilgaard (1984) the criminal gene
Aim:
Alice Theilgaard (1984) wanted to see if criminals had a particular gene that could be
responsible for their criminal behaviour.
Procedure:
Took blood samples of over 30,000 men born in the 1940s.
From these initial blood tests two chromosome abnormalities were found;
XXY Chromosome pattern
XYY Chromosome pattern
Out of 30,000 tested 16 had XXY and 12 had XYY

6. The men were interviewed by a social worker about their background and their criminal history
and given an intelligence test.
A personality test was given to see if they displayed more aggression than the XY Males.
Interviewing and Research Bias
The Social workers did not know the aim of the study. By doing this was able to avoid Research
Bias (find out what they want to find out).

7. Results – Theilgaard Found…
XYY – Had slightly lower intelligence and tended to be more aggressive than normal.
This could be evidence for a criminal gene if aggression is a sign of criminality. (but not all
criminals are aggressive).
There were far more similarities between the XXY and XYY males than there were differences.
No solid evidence of a criminal gene was found.

8. Evaluation Strengths
All tests and interviews conducted by independent Social Worker (No researcher Bias)
Used a vast range of tests to measure different aspects of men's lives, backgrounds and
personality.

9. Weakness
Only a small sample of men used (16 and 12) this is because XYY occurs in estimate 1 in 1000.
Only 12 males tested can not be sure that all XYY are more aggressive and lower intelligence.
(Can’t be generalised).
XYY are verily average so can not be picked out from other males easily. Not inherited so can
not be tracked easily.
Link between XYY and aggression is only a Correlation not a cause.
The most obvious reason for aggression males were lower intelligence and delayed speech.
Could have made school difficult for them. Leading to frustration and aggression in childhood
leading through to adulthood.

10. Similar studies
Simple fact is that from many studies that have been done.
Not all criminals are XYY and not all XYY males are Criminals.
So you can not be sure that chromosome abnormality causes criminality or all criminals would
be XYY.
Dilemma
What would happen if XYY chromosome was identified?
Would all XYY be monitored in case they committed a crime?
Could this have lead them to crime as a result of being labelled.