AO1 Twin Studies
Monozygotic (MZ) twins share all of
their genetic information whilst dizygotic twins (DZ) share 50%. If MZ twins
are more alike in terms of aggressive behaviour than DZ twins, then this should
be due to genes rather than environment. It is much harder to study aggressive
behaviour than criminal behaviour, so few studies have tried to measure
aggressive behaviour directly. However, Coccaro did use twins to study
aggressive behaviour and found that 50% of the aggression towards to each other
could be attributed to genetic factors.
AO2
However one problem when studying
twins is that twins will often be brought up together in a similar way, so it
is hard to say whether the concordance rates are caused by genetic or
environmental factors. To solve this problem psychologists can carry out
adoption studies where they observe the levels of aggressive behaviour in
adopted children and their biological parents. If there is a high correlation
between aggression levels of the two you can say it is genetics not the
environment that caused this aggression. Hutchings and Mednick (1975) studied
14,000 adopted children and found that a high proportion of boys with criminal
convictions had biological parents with criminal convictions too, suggesting a link
between aggression and genetics.
There are a number of studies that have
used twins. Miles and Carey (1997) looked at 24 studies involving twins or
adopted children. They found that genetic influences were very important in
these studies. Another meta-analysis carried out by Rhee and Waldman (2002)
also found that genetic factors were very important in aggressive behaviour.
IDA
The genetic argument is also supported by
real-life examples of violent individuals. Reggie and Ronnie Kray were
identical twins who were also violent criminals and were responsible for a
series of murders in the 1960s. Robert and Stephen Spahalski were twin brothers
in the USA. They were raised together in childhood but separated afterwards.
Both became serial killers.
The Monoamine
Oxidase A (MAOA) gene attracted interest when it was found that mice lacking
the gene were excessively aggressive. Monoamine oxidase A is an enzyme
that breaks down important neurotransmitters in the brain,
including dopamine, norepinephrine, and serotonin. The enzyme is regulated by
the monoamine oxidase A gene (MAOA). Humans have various forms of the
gene, resulting in different levels of enzymatic activity. People with
the low-activity form (MAOA-L) produce less of the enzyme, while the
high-activity form (MAOA-H) produces more of the enzyme.
Brunner found
evidence that the MAOA deficiency is genetic. Brunner conducted a study on a
large Dutch family, in which all the males had a mutant form of the MAOA gene.
All had borderline retardation and reacted aggressively when angry, fearful or
frustrated, suggesting that abnormal MAOA activity is associated with
aggression. Brunner’s research however, is extremely limited as it is a case
study, meaning it used a small sample. Although a case study produces data rich
in detail, the conclusions cannot necessarily be applied to a wider population.
Also, there are other reasons that could explain the disproportionate amount of
aggressive males in the family studied; examples being dispositional factors
and the pressure of maintaining the family’s reputation.
Moffitt also
conducted research into the MAOA deficiency. Researchers performed a
longitudinal study on 442 NZ males from birth to age 26, recording which
participants suffered abuse as children and activity of participant’s MAOA
gene. It was found that those who had suffered abuse, and had the deficient
MAOA gene were 9x more likely to indulge in antisocial behaviour, including
aggression. Participants who had not been abused, but carried the high activity
form of MAOA were not more aggressive than anyone else. This suggests that the
MAOA gene has some influence on aggression, but it is sensitive to social
experiences in early development. This implicates the idea of a diathesis
stress model; some people are genetically pre-disposed to be aggressive, but it
will only manifest itself when triggered by an environmental experience.
Moffitt’s research used a large sample over a long period of time, suggesting
that the conclusions made can be applied to the wider population.
However, as the research was conducted in NZ, it may have produced a culture
bias. This may mean that the conclusions cannot be applied to people living in
different societies.
However, the
impact that genes alone have on aggression has been elusive. It seems that our
genes affect us most when combined with certain environmental stimuli e.g. in
Moffitt’s research a genetic deficiency was only made apparent when triggered
by abuse. This suggests that the genetic approach is reductionist, as it
ignores the social and environmental factors that impact on our aggression. It
is likely that the interactionist approach proposed by Moffatt is the most
valid, as it accounts for genetic factors (MAOA) as well as environmental
factors (abuse).
The approach is
also deeply deterministic, in that it states that we are passive victims of our
own genes, disregarding our free will in choosing how to behave. Also,
many studies into genetics have used animals, as they have quicker breeding
cycles, so intergenerational effects can be observed sooner. This however,
presents a variety of ethical issues. For example, animals cannot provide
informed consent, and cannot tell us if they have been harmed by the research.
Also, it is extremely difficult to generalise conclusions made about the
genetics of animals to humans, we are simply too different.
Also, measuring
aggression in such studies can be difficult. How do we operationalize
aggressive acts? For example, in Moffatt’s study, how did researchers determine
whether a participant was behaving aggressively or not? This could present a
problem of inter-study reliability as measures are not consistent.
Many of the studies in this area have
focused on individuals convicted of violent crime. However, these
results only represent a small minority of those who are regularly involved in
aggressive behaviour, as many violent attacks do not results in a conviction.
These individuals may also not be habitual offenders; they may be usually calm
people who were designated as violent for a one-time offense. This may explain
why so many studies have found little or no evidence of heritability for
violence.
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