A recent brain imaging study provides evidence that individuals accused of murder tend to have smaller volumes in specific brain regions related to emotion and decision-making. The research suggests that these physical brain differences are closely tied to psychopathic personality traits, particularly in people who methodically plan out their crimes. These findings were published in the peer-reviewed journal Aggression and Violent Behavior.
The human brain has specialized areas that help regulate emotions and guide social behavior. The amygdala is a small structure located deep in the brain that plays a major role in processing emotions. It helps people recognize fear, learn from negative experiences, and make moral decisions. A poorly functioning amygdala might prevent someone from feeling normal emotional responses during social interactions.
Another important area is the orbitofrontal cortex, located in the front of the brain just above the eyes. This region is involved in weighing risks and rewards. It helps people control their impulses and understand the negative consequences of their actions. Past criminal psychology literature links these brain areas to aggressive and antisocial behaviors.
Adrian Raine, a professor of criminology, psychiatry, and psychology at the University of Pennsylvania, wanted to explore these structures in an unstudied group. Raine, who is also the author of the book The Anatomy of Violence, helped conduct the research to fill a gap in the scientific literature. “We had previously published the first-ever functional brain imaging study on murderers back in 1994, but since then there has been only one other small-scale imaging study on pre-trial murderers, and none that investigated structural imaging,” Raine said.
The collaboration for the project began rather organically during a trip abroad. “The study came about unexpectedly,” Raine noted. “I happened to have dinner with Chenbo Han when I was visiting Nanjing and we exchanged interests.”
Han evaluated all forensic cases in the Jiangsu Province, which made it possible to recruit subjects. “Chenbo was the lead forensic psychiatrist in Nanjing and we realized we had a lot of interests in common,” Raine said. “Sometimes research studies are planned, but other times they take place a bit by chance.”
Most prior brain imaging studies looked at people who had already been convicted and who had spent years in prison. Spending time in a restrictive and stressful prison environment can actually alter brain structure over time. To avoid this complicating factor, the authors focused on suspects who were awaiting trial.
The timing of the brain scans was another important factor. For post-trial prisoners, there is usually a delay of several years between the time of the homicide and the brain scan. These extra years can add significant errors to the imaging data. In this pre-trial sample, the delay was relatively short, typically just a few months, which provides a more accurate snapshot of the brain.
These scientific findings are increasingly relevant to the emerging field of neurolaw. Neurolaw examines how brain science intersects with the legal system. Brain imaging is sometimes introduced during the sentencing phase of death penalty cases to argue for reduced punishments.
Raine highlighted the real-world impact of this type of biological data. “Murderers have a different brain structure to the rest of us – their brains are physically different in a way that predisposes them to violence,” Raine said. “These findings have potential forensic implications in the court room.”
Medical evidence of this nature creates a complex legal dilemma for judges and juries. “If murderers have brain impairments that predispose them to violence, are they truly responsible for their actions?” Raine asked. “On the one hand that might lead us to be more lenient in sentencing.”
However, acknowledging these deep-seated physical traits could also work against the defendant. “But on the other hand, if we cannot treat their brain impairments, should this lead us to longer sentences to protect society?” Raine explained. Establishing a reliable scientific baseline for brain function in offenders helps the legal system evaluate these medical claims objectively.
The study included 87 participants from the Jiangsu Province of China. The sample consisted of 37 people accused of murder who were undergoing forensic psychiatric evaluation while waiting for their trials. The control group included 50 non-violent individuals from the exact same community who had no history of criminal offending. All participants were of Han ethnicity, and both male and female subjects were included in the research.
All participants underwent a structural magnetic resonance imaging scan. This type of MRI scanner creates highly detailed, static pictures of the brain’s anatomy. A computer program was used to measure the exact size of the lateral and medial sections of the orbitofrontal cortex. Two trained technicians manually traced the boundaries of the right and left amygdala to calculate their total volume.
Forensic psychiatrists evaluated the participants to measure their personality traits. They used the Psychopathy Checklist-Revised, which is a standard interview assessment designed to measure psychopathy. Psychopathy is a personality construct characterized by a lack of empathy, shallow emotions, impulsivity, and a failure to feel guilt.
For the murder suspects, the psychiatrists also reviewed police files and family reports to rate the degree of planning involved in the crimes. The rating scale ranged from one for a completely impulsive act to four for a completely planned homicidal act. To make sure their results were accurate, the scientists controlled for several outside variables. These variables included age, gender, head injuries, substance use, broken homes, cognitive functioning, and overall skull size.
The researchers found measurable brain differences between the two groups. The murder suspects had a smaller lateral orbitofrontal cortex, which is the outer section of this frontal brain area. They showed a 4.9 percent volume reduction in this region compared to the non-violent control group. The medial orbitofrontal cortex, which is closer to the center of the brain, did not show any size differences.
The scientists did not anticipate this specific anatomical difference in the outer frontal cortex. “We had not expected to see volume reductions in the lateral orbitofrontal cortex, but we were intrigued to find two studies showing that when healthy individuals accidentally kill an innocent victim in a video game, they show increased activation in this very same brain region,” Raine said. This area seems to process the emotional weight of causing harm.
A physical reduction in this specific section might limit a person’s ability to feel remorse for harming innocent people. “Furthermore, those who feel most guilty about their wrongful killing showed the greatest activation,” Raine continued. “So it seems that when this brain region is structurally impaired, it might make people feel less guilty about wrongfully killing someone, and this may take the brake off killing someone.”
The murder suspects also displayed smaller overall amygdala volumes. Their amygdalas were 5.9 percent smaller than those of the control group. This size variation was especially prominent in the left hemisphere of the brain. The scientists used advanced surface mapping to look closer at the exterior structure of the left amygdala.
They found that the physical shrinkage was localized to specific internal clusters known as the central, lateral, and basolateral nuclei. These amygdala sections are biologically essential for learning from fear and avoiding painful outcomes. Underdevelopment in these zones is often linked to aggressive and disruptive behavior problems in children and adults.
The data provides evidence that the level of preparation involved in a crime relates to brain anatomy. Murder suspects who planned their homicides had a 14.3 percent smaller amygdala than those whose crimes were mostly impulsive. Planned crimes are generally considered to be calculated and less driven by sudden bursts of emotion.
The connection between careful preparation and brain size stood out to the research team. “Similarly, we were struck by the fact that it’s the murderers who planned their homicides who had the reduced amygdala volumes – so it’s the more ‘cold-blooded’ murderers who have blunted emotions as indicated by this volume reduction,” Raine said.
The researchers observed strong connections between brain volume and psychopathic traits. Higher scores on the psychopathy assessment were associated with smaller amygdala sizes in both the left and right hemispheres. “Perhaps not surprisingly, murderers with high scores on psychopathy were most likely to have a volume reduction in the amygdala,” Raine noted.
This relationship was most pronounced for the affective features of psychopathy. Affective features reflect the emotional shallowness and lack of remorse typically seen in psychopathic behavior. The researchers performed follow-up tests to see if these patterns held true for all participants.
A significant portion of the participants had a diagnosis of schizophrenia. Because of this, the researchers included schizophrenia diagnosis as a controlled variable in their statistical models to ensure it did not skew the results. They separated the murder suspects into those with and without schizophrenia.
In both sets of murder suspects, higher psychopathy scores still predicted reduced amygdala sizes. “We were surprised that the findings for the amygdala were quite strong and appeared robust, replicating from murderers with a comorbid diagnosis of schizophrenia to murderers lacking this diagnosis,” Raine said. This indicates that schizophrenia was not the cause of the brain variations.
The scientists applied the exact same rigorous testing to the non-violent control group. They checked if psychopathic traits predicted brain size in everyday citizens who had no history of violence. The researchers found a similar trend linking affective psychopathic traits to left amygdala reductions in typical individuals. This highlights that the relationship between brain structure and personality exists even outside of criminal populations.
The authors also ran statistical tests to see what was driving the main group differences. They found that the higher levels of psychopathy in the murder group explained the smaller brain structures. When the researchers mathematically adjusted the results to equalize psychopathy scores across everyone, the brain differences between the murderers and the control group vanished. This suggests that the brain size reduction is primarily linked to psychopathic traits rather than the legal act of murder itself.
Readers should avoid assuming that these anatomical differences are a definite cause of criminal action. Brain imaging provides statistical group trends, but it cannot predict or explain exactly what any individual person will do in the real world. Many people with small amygdalas never commit crimes, and not all criminals have brain differences. These structural variations simply act as one potential risk factor for aggressive behavior.
The study has a few acknowledged limitations. The total sample size of 87 people is relatively small. Also, the researchers used a standard 1.5 Tesla MRI scanner. This equipment produces less detailed images than newer, stronger scanning devices, meaning the exact boundaries of the tiny sub-sections within the amygdala are only approximations.
The geographic focus is another factor to consider. “This is a study on murderers from China and at the present time findings, while suggestive, cannot as yet be generalized to other countries,” Raine said. “The sample was not large, but it is quite typical of forensic brain imaging samples and findings are consistent with theoretical expectations.”
In this specific Chinese sample, the overall severity of psychopathy was relatively low. The current findings apply to individual differences in general psychopathic traits, rather than to extreme or clinical forms of psychopathy. Future studies should aim to include larger numbers of female murder suspects, which would allow scientists to see if the anatomical patterns differ between genders.
The long-term goal of this science is to find ways to reduce violence. “I’m really interested in how we can change the brain to change behavior in a benign way in samples like this,” Raine explained. Nutrition might offer one pathway.
“For example we have conducted randomized controlled trials showing that omega-3 supplementation can reduce aggression by about 30%, and that this applies all the way from prisoners to children in the community,” he said. “Biology is not destiny, and we can change the brain to change violent behavior,” Raine concluded.
The study, “Reduced amygdala and lateral orbitofrontal volumes in pre-trial murderers: The role of psychopathy,” was authored by Adrian Raine, Jules Dugre, Chenbo Han, Robert Schug, and Jianghong Liu.
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