Previous studies have repeatedly dealt with the psyche of murderers in comparison to other people, but have often shown crucial gaps. In order to close these gaps, a group of scientists carried out the most comprehensive study to date - and recognized key deviations in the “grey matter.”
Studies with Positron Emission Tomography (PET) scans from the 1990s concluded that in certain brain regions, such as the prefrontal cortex or the amygdala, there was reduced activity in murderers.
However, these studies only included people that were found not guilty due to mental illness. The possibility that these diseases could be the basis for deviations in the brain was ignored. Instead, they concluded these instances as "murderous intentions". The authors of the current study, therefore, doubt the validity of older studies: "These results are not sufficient to distinguish murder from other violent crimes or mental illness", they argue.
Contrary to earlier research approaches that included non-imprisoned people in the control group, the new study focuses only on imprisoned people. The 808 male study participants were assigned to three different groups: Murderers (203 participants), violent criminals who did not commit murder (475 participants) and non-violent offenders (130 participants).
People who suffered from mental illness or lost consciousness for more than two hours due to traumatic brain injuries were excluded from the study. Also not included in the study were crimes that were due to accidents and participants without direct involvement in a crime.
In addition to MRI scans, researchers used data such as drug use, age, time in prison and IQ. Compared to violent criminals and non-violent offenders, there were clear differences in the brain among the murderers, which persisted even after checking the different control data. Since there were no differences in the brain between the brains of violent criminals and other prisoners, the researchers came to the conclusion that murderers have unique neuroanatomy.
The research team identified deficits in different regions of the prefrontal cortex, the insular cortex, the cerebellum, and the posterior cingulate cortex. The researchers explained: "In murderers, we were able to detect a reduced amount of grey matter in the brain regions that are important for factors such as affective behavior, social cognition or the strategic handling of the other.
The scientists point out, however, that this study is also only of limited significance. Despite extensive analysis, they assume the possibility that other important parameters may have been overlooked. They mention, for example, that their investigations did not focus on impulsivity, which could mean that murderers act only more impulsively than other people.
Likewise, the study only included brain scans at a point in life and therefore does not provide any information as to whether a person was born with these deficits or whether they developed over time.
The study authors also made it clear that their investigations should by no means be interpreted as considerations of biological determinism: "Under no circumstances should our investigations be used to identify murderers on the basis of the evaluation of brain data. Nor can they be used to predict future murderous behavior."
Due to the scope of the study, the researchers are convinced that their investigations form a solid basis for the clarification of further details. In the future, they want to work on a precise representation of the measured differences and the networks they link in the brain.
Thus, researchers leave it to others to explore how and why these neuroanatomical changes occur and how to deal with them. However, they do not expect these questions to be answered in the near future.
The researchers concluded: "By emphasizing the crucial role of mental health and personal development in the most extreme form of violence, our work represents an incremental step towards making our society safer.''
Sajous-Turner A et al., Brain Imaging and Behavior 2019; https://doi.org/10.1007/s11682-019-00155-y