Genetic factors in aggression
MAOA: an enzyme that, among other things, regulates the metabolism of serotonin in the brain
Depth:
Genetic explanation for aggression suggests that the likelihood of behaving in a particular way is
determined by a person’s genetic make-up i.e., it is inherited from parents.
A specific gene, the MAOA gene has been associated with aggressive behaviour.
MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are
associated with impulsive and aggressive behaviour.
In the 1980s, a study of a Dutch family found that many of its male members behaved in a
particularly violent and aggressive manner, and a large proportion had been involved in serious
crimes of violence including rape and arson. These men were found to have abnormally low levels
of MAOA in their bodies, and a later defect in this gene was later identified.
A second study, linking MAOA to aggressive behaviour involved 500 male children.
Researchers discovered a variant of the gene associated with high levels of MAOA (MAOA-H) and a
variant associated with low levels of MAOA (MAOA-L).
Those with the MAOA-L variant were significantly more likely to grow up to exhibit aggressive
behaviour but ONLY if they had been maltreated as children.
Children with the MAOA-H variant who were not maltreated, did not display aggressive behaviour.
MAOA-L is much more frequent in populations with a history of warfare, with about two-thirds of
people in these populations having this version of the gene. This has led to it being referred to as
the ‘warrior gene’.
MAOA-L variant has been linked to high levels of aggressive behaviour as it results in low activity
of the MAOA enzyme and low levels of serotonin.
Bredth:
Twin studies were carried out to investigate the effects of genes – research found that direct
physical assault concordance rates of 50% for MZ twins and 19% for DZ twins – this tells us that
aggression is caused by genes rather than the environment.
Adoption studies studied over 14,000 adoptions in Denmark and found a significant number of
adopted boys with criminal convictions had biological parents with convictions for criminal
violence, strengthening the genetic explanation as adoption studies rules out the influence of the
environment.
EVALUATION
WEAKNESS
P – It is difficult to establish genetic contributors to aggression because more than one gene usually
contributes to a given behaviour
E – As well as genetic factors, there are many non-genetic (environmental) influences on the manifestation
of aggression such as Social Learning Theory.
E – These influences may interact with each other. Genetic factors may affect which environmental factors
have an effect (gene-environment interaction).
MAOA: an enzyme that, among other things, regulates the metabolism of serotonin in the brain
Depth:
Genetic explanation for aggression suggests that the likelihood of behaving in a particular way is
determined by a person’s genetic make-up i.e., it is inherited from parents.
A specific gene, the MAOA gene has been associated with aggressive behaviour.
MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are
associated with impulsive and aggressive behaviour.
In the 1980s, a study of a Dutch family found that many of its male members behaved in a
particularly violent and aggressive manner, and a large proportion had been involved in serious
crimes of violence including rape and arson. These men were found to have abnormally low levels
of MAOA in their bodies, and a later defect in this gene was later identified.
A second study, linking MAOA to aggressive behaviour involved 500 male children.
Researchers discovered a variant of the gene associated with high levels of MAOA (MAOA-H) and a
variant associated with low levels of MAOA (MAOA-L).
Those with the MAOA-L variant were significantly more likely to grow up to exhibit aggressive
behaviour but ONLY if they had been maltreated as children.
Children with the MAOA-H variant who were not maltreated, did not display aggressive behaviour.
MAOA-L is much more frequent in populations with a history of warfare, with about two-thirds of
people in these populations having this version of the gene. This has led to it being referred to as
the ‘warrior gene’.
MAOA-L variant has been linked to high levels of aggressive behaviour as it results in low activity
of the MAOA enzyme and low levels of serotonin.
Bredth:
Twin studies were carried out to investigate the effects of genes – research found that direct
physical assault concordance rates of 50% for MZ twins and 19% for DZ twins – this tells us that
aggression is caused by genes rather than the environment.
Adoption studies studied over 14,000 adoptions in Denmark and found a significant number of
adopted boys with criminal convictions had biological parents with convictions for criminal
violence, strengthening the genetic explanation as adoption studies rules out the influence of the
environment.
EVALUATION
WEAKNESS
P – It is difficult to establish genetic contributors to aggression because more than one gene usually
contributes to a given behaviour
E – As well as genetic factors, there are many non-genetic (environmental) influences on the manifestation
of aggression such as Social Learning Theory.
E – These influences may interact with each other. Genetic factors may affect which environmental factors
have an effect (gene-environment interaction).
