biological explanations of aggression

•XYY karotype – research by Court-Brown suggested men with
this extra Y chromosome had increased likelihood of
aggressive behaviour (and should be locked up). This link was
later retracted.
• How aggression is passed from generation to generation Nelson – selective breeding can lead to more aggressive
behaviour in animals
•Theilgaard researched the personality traits of XYY men
compared to XY men and XXY men.
• XYY are about 1 in 1000men and only characteristic
associated with it seems to be height.
• In her study she showed that XYY tended to give more
aggressive interpretations of images, but this doesn’t mean
they would act aggressively.
Twin studies have generally focused on criminal
behaviour using MZ and DZ twins –
Coccaro – using adult twins found that nearly 50% of
variance in direct aggressive behaviour was due to
genetic factors.
Adoption studies – if a positive correlation is found
between aggressive behaviour in adopted children and
in their biological parents, a genetic effect is implied. If
positive correlation found between adopted child and
rearing family – environmental effect implied. A study
of over 14000 adoptions in Denmark found a significant
number of adopted boys with criminal convictions had
biological parents (especially father) with criminal
convictions – Hutchings and Mednick
Meta analysis – Miles and Carey 24 twin and
adoption studies.
Used self reports and some observations
Results suggested strong genetic influence – upto
Age differences were important , genes and family
environment influential in youth but influence of
rearing environment decreased with age.
A gene for aggression
The role of MAOA no individual gene has been found for
aggression but a gene has been found for producing a
protein MAOA that is associated with aggressive behaviour.
MAOA regulates serotonin and low levels of this are
associated with impulsive and aggressive behaviour.
Brunner – study of Dutch family where male members
behaviour was particularly violent and aggressive – also had
very low levels of MAOA.
Caspi – study of 500 male children found 2 variants of the
gene – one associated with high levels of MAOA and one
with low levels. Those with low level significantly more
likely to grow up showing anti-social behaviour IF
maltreated as children (no difference if not maltreated)
showing a gene – environment interaction.
May be no gene for violent crime but inherited
temperament or personality may place some at
more risk of committing violent crime.
Adoption studies show highest rate of criminal
violence in adopted children is when biological and
adoptive parents have a history of violent crime. In
other studies genetic influence only significant in
cases of property crime not violent crime.
•Very difficult to determine role of genes
•More than one gene normally contributed to
a given behaviour
•Lot of non-genetic influences
•The two may interact with each other –
gene-environment interaction.
•Problems of assessing aggression
Studies using parental and self reports of aggressive
behaviour show higher genetic contribution than those
using observations - methodological problems.
Studies often fail to distinguish between violent and nonviolent crime therefore difficult to untangle genetic
factors in aggressive behaviour and between habitual and
one off violence. Walters – meta analysis only showed
low to moderate correlation between heredity and crime,
and modern better designed studies show lower
Problems of sampling. Studies have focused on
individuals convicted of violent crime, relatively few
are actually convicted so may not be representative
of those involved in aggressive behaviour. Some
convicted may be one off offenders rather than the
most persistent
Real-World Application
•Despite weak evidence for biological base suggestions put
forward to ‘treat’ those predisposed to violence e.g genetic
engineering, and chemical castration. Ethical consequences
of labelling someone a threat based on their genetic
inheritance show why an awareness of the limitations of
these studies is very important
•Lot of studies into aggressive behaviour has been done
on animals often using selective breeding programmes
and ‘knockout’ techniques. Young – claimed to have
identified a genetic mutation that caused violent
behaviour in mice. A counterpart of this gene does
appear in humans although precise function unknown.
•Ethical issues involved with animal testing.
•Problem of generalising to human behaviour.

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