A relatively new way of
looking for differences between people is to examine patterns of brain
activity through the use of imaging techniques. There are three basic
procedures that are currently used:
PET (positron emission tomography)
PET stands for Positron Emission Tomography.
PET scans can be used to examine the relationship between the metabolic
activity in the brain and mental processes.
A small amount of harmless radioactive material is
injected into the participant which bonds to a substance such as
glucose. This is called a tracer. As the brain uses
glucose as energy, the areas of the brain which are most active absorb
it. The glucose is broken down but the radioactive material
remains and it emits positively charged particles called positrons which
are picked up by the scan. This information can be read by a
computer which produces coloured images of the level of activity
occurring throughout the brain. PET scanning is the
most established of brain-imaging techniques, but requires massive
MRI (magnetic resonance
imaging) uses a combination of powerful magnets and radio pulses to
measure changes in oxygen levels and hence blood flows in the brain.
(magneto-encephalography) uses very sensitive sensors to pick up the faint
magnetic fields generated by active nerve networks.
The aim of the study was
to look at direct measures of both cortical and subcortical brain
functioning using PET scans in a group of murderers who have pleaded not
guilty by reason of insanity (NGRI). The expectation was that the murderers would show
evidence of brain dysfunction in their prefrontal cortex as well as in
other areas that are thought to be linked to violent behaviour.
The method is a laboratory
experiment using an independent measures design. The main independent variable is whether the participant had
committed murder or not. The
dependent variables were the results of the PET scans.
This design is sometimes referred to as a quasi or natural
experiment because the researchers do not have total control over the
independent variable. That
is, the participants cannot be randomly placed in one of the two
The study used PET scans
to examine the brains of 41 people (39 males and 2 females) who were
charged with murder and were pleading Not Guilty for Reasons of Insanity
(NGRI), and compared them with 41 controls.
All the NGRIs were referred to the imaging centre for legal
reasons, such as to obtain evidence for the defence.
The reasons for the referrals were:
Schizophrenia (6 cases)
Head injury or organic damage (23)
Drug abuse (3)
Affective disorder (2)
Hyperactivity or learning difficulties (3)
Personality disorder (2)
The mean age of the NGRIs
was 34.3 years. The controls
were selected to match for age and sex and the six NGRIs who were
diagnosed as schizophrenic were matched with six other people with the
same diagnosis but no history of murder.
All the controls were further screened for their mental and
All offenders were in
custody and were kept medication free for the two weeks before brain
scanning. The control group
were also medication free.
All of the participants
were injected with a glucose tracer, required to work at a continuous
performance task that was based around target recognition for 32 minutes,
and then given a PET scan. The
NGRIs were compared with the controls on the level of activity (glucose
metabolism) in right and left hemispheres of the brain in 14 selected
areas. The researchers looked
at activity in six cortical areas (part of the cerebral cortex which is
the outermost layer of nerve tissues of the cerebral hemispheres) and
eight subcortical areas (brain structures below the cortex);
Lateral prefrontal (involved with thinking and
Corpus callosum (link between hemispheres)
Medial prefrontal (involved with thinking and
Amygdala (part of the limbic system governing
Parietal (responsible for integration of
sensory input and formation of abstract concepts)
Medial temporal lobe and hippocampus (involved
in learning and memory)
Thalamus (also involved in learning and memory)
Temporal (hearing, memory, emotion, language)
Putamen (often linked to mental disorders)
Globus pallidus (often linked to mental
Midbrain (often linked to mental disorders)
Cerebellum (often linked to mental disorders)
The cerebral cortex is commonly described in terms of four areas or
lobes; the prefrontal, parietal, temporal, and occipital.
In this study, compared to the controls, the NGRIs were found to
have less activity in their prefrontal and parietal areas, more activity
in their occipital areas, and no difference in their temporal areas.
The results from the subcortical
areas found less activity in the corpus callosum (which joins the two
halves of the brain; see Sperry). They
also found an imbalance of activity between the two hemispheres in three
other subcortical structures. In
the amygdala and the hippocampus, compared to the controls, the NGRIs had
less activity in the left side and more activity in the right side.
Also, in the thalamus the NGRIs had more activity in the right
side, though no difference in the left side.
Raine et al. argue that
their research supports previous findings about the role of certain brain
structures in violent behaviour. They
suggest that the difference in activity in the amygdala (which is part of
the limbic system) can be seen to support theories of violence that
suggest it is due to unusual emotional responses such as lack of fear.
The authors also comment on the differences in corpus callosum
activity between the NGRIs and the controls, and suggest this can be
matched up to evidence of people with a severed corpus callosum which show
they can have inappropriate emotional expression and an inability to grasp
long-term implications of a situation.
However it is important to note that Raine et al. are cautious
about the implications of their findings.
They note that the findings:
Cannot be taken to show that violence is only caused by
Do not show that NGRIs are not responsible for their
Do not say anything about the causes of the brain
Cannot be generalised from NGRIs to other types of violent
Cannot be generalised to other types of crime.
Evaluation of the Procedure/Method
The main methodological
strength of this study is the amount of control the researchers had over
the procedure. For
example they used a control group who were matched on variables such as
age and sex and they were screened for their physical and mental health.
It can also be argued that
PET scans are useful because we no longer have to wait for a person to die
before we can examine their brain.
The scan allows for a wide range of non-intrusive studies.
However the study does
have methodological weaknesses.
For example PET scans are still being developed and therefore the
data should be treated with caution.
The task used by the
participants before the scan is a general activity task and has no bearing
on violent acts or even the decision to be violent.
Therefore the validity of the task could be questioned.
The major methodological
weakness is related to the design of the study.
It has already been noted that the design was a type of natural
experiment, which means that the researchers could not randomly allocate
participants to either of the two conditions. Therefore we have to be careful in making cause and effect
statements. It is possible
that any differences in brain activity between the two groups could have
been caused by other factors. We
can only say that the study shows a correlation and not a causal
example, it could be that the differences between the two groups might
have been a result of the crime and its consequences rather than a cause
Evaluation of the Explanation
A main strength of the
researchers argument that dysfunctions in certain brain structures can
lead to violent behaviour is the possibility of treatments for violent
people. Perhaps drugs
could be used to alter the functioning of brain structures thereby
reducing a personís possibility of committing violent crimes.
However this argument
raises many ethical dilemmas. We
would be wise to have reservations about any suggestions that it is
possible to identify potential murderers.
It is also important to
question the notion that all murderers are violent.
NGRIs are not necessarily charged with murder because of a violent
act. It is possible to
murder someone with poison, which it could be argued is not violent. Dr Shipman might be an interesting example of murder
without violence. This of
course depends upon your definition of violence.
The main criticism of this
study refers to the issue of reductionism.
Studies like this one have been criticised for being biologically
reductionist in that they attempt to explain complex behaviour as a
consequence of brain functioning. By doing so this ignores the many other possible reasons why
a person may act violently. Brain
functioning is possibly just one factor why a person may act violently.
Other factors that must be taken into account include a persons
social background, their role models, psychological predispositions,
learned responses and so on.
Buchsbaum, M & LaCasse, L. (1997) Brain abnormalities in murderers
indicated by positron emission tomography. Biological Psychiatry, 42
(6), 495 - 508