a) Genetic influence
b) Biochemical influence; hormones and neurotransmitters
c) Brain structure influences
a) Genetics
To try and disentangle nature from nurture researchers have used twin and adoption
studies and studies of violent populations.
XYY karotype – research by Court-Brown suggested men with this extra Y
chromosome had increased likelihood of aggressive behaviour. This link was later
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
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 - reductionist
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. See text book for e.g.s
Studies often fail to distinguish between violent and non-violent 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 correlations
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.
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
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
a) Ethical issues involved with animal testing.
b) Problem of generalising to human behaviour. Conclusions from animal
testing might be limited when generalised to humans as aggression has
cognitive implications which animals may not have.
Practical application - Anger management courses.
Determinism – Biological explanations of aggression are deterministic as it
means people do not have free will over their behaviour. One implication is
that if people are biologically determined then people cannot be held
responsible for their aggressive behaviour.

Biological explanation of aggression genetics.doc