Brain Abnormalities in Murderers Indicated by PET scans

A relatively new way of looking for differences between people is to examine patterns of brain activity through the use of imaging techniques of PET scans (positron emission tomography). A small amount of harmless radioactive material is injected into the participant which bonds to a substance such as glucose. 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 investment.   

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 a 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 conditions. 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),Epilepsy (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 physical health.  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 in right and left hemispheres of the brain in 14 selected areas. The researchers looked at activity in six cortical areas and eight subcortical areas; Cortical Areas, Subcortical areas, Lateral prefrontal, Corpus callosum, Medial prefrontal, Amygdala, Parietal, Medial temporal lobe and hippocampus, Occipital, Thalamus, Temporal, Putamen, Cingulate, Globus pallidus, Midbrain and cerebellum.

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 . 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 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 behaviour;Do not show that NGRIs are not responsible for their actions;Do not say anything about the causes of the brain differences;Cannot be generalised from NGRIs to other types of violent offenders; and Cannot be generalised to other types of crime.

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. PET scans are useful because we no longer have to wait for a person to die before we can examine their brain allowing for a wide range of non-intrusive studies. 

However, 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, 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 relationship. 

A main strength of the researcher’s 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 person’s 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.   

The main criticism of this study refers to the issue of reductionism. Studies like this one have been criticised for being biologically reductionist by doing so this ignores the many other possible reasons why a person may act violently. Other factors that must be taken into account include a person’s social background, their role models, psychological predispositions, learned responses and so on. The key findings from this study are that murderers pleading NGRI are characterised by; reduced glucose metabolism in the prefrontal cortex, the parietal cortex, and the corpus callosum. Abnormal asymmetries of activity in the amygdala, thalamus, and the hippocampus.