By scanning
the brains of hundreds of convicted murderers, a recent study has zeroed in on
significant differences in the gray matter of homicide offenders, compared with
that of people who have carried out other violent crimes.
In 2016, there were an estimated 17,250murders in the United States.
Homicide, of course, has a devastating impact on individuals and
society at large.
As such, it is essential to study the biological, psychological,
and social basis of these terrible acts.
Although earlier studies have looked at how a murderer's brain
might differ from that of a non-murderer, they have often been flawed.
A group of scientists recently set out to fill in some of the
gaps, and they designed the largest study of its type, to date. The researchers
have published their findings in the journal Brain Imaging and Behavior.
Flaws in earlier work
Earlier studies using PET scans, carried out in the
1990s, concluded that homicide offenders' brains demonstrated reduced activity
in a number of brain regions.
These include parts of the prefrontal cortex — which is a region
that is important for moderating social behavior, among many other things — and
the amygdala, which plays a vital role in processing emotions.
Although the findings were interesting, the researchers had
exclusively enrolled participants who had been found "not guilty by reason
of insanity." Therefore, any of the differences that the scientists
measured could have been due to mental illness or brain injury, rather than
homicidal tendencies.
Other, later studies have investigated the brains of violent
individuals with conditions such as schizophrenia. These
researchers have found changes in similar brain regions, but they face the same
issues. As the authors of the new study explain:
"They are not
sufficient to discriminate homicide from other violent outcomes or from other
psychiatric disorders."
A new approach
Many of the earlier studies used non-incarcerated individuals as
a control group, which is far from ideal. To remedy this, in their latest
project, the authors only recruited inmates.
In total, the scientists took data from 808 adult male inmates;
each participant fitted into one of three groups:
homicide offenders (203 individuals)
violent offenders who had not carried out a homicide (475
individuals)
nonviolent or minimally violent offenders (130 individuals)
Importantly, they excluded individuals with a psychotic disorder
and any who had lost consciousness for more than 2 hours as the result of
a traumatic
brain injury.
The scientists did not include any person who had been convicted
of a crime that could have involved an accidental death. They also excluded
participants who had not been directly involved in the offense.
Alongside MRI scans, the researchers
considered other details, including information about substance use,
participants' age, and how long they had been in prison. They also estimated
each participant's IQ.
Compared with the violent and nonviolent offenders, the brains
of the homicide offenders were significantly different; and this difference
remained apparent, even after the scientists controlled for the factors
mentioned above.
Interestingly, there were no significant
differences between the brains of violent and nonviolent offenders. It seems
that the neuroanatomy of a murderer is unique.
Where were the differences?
The scientists saw deficits in a range of brain regions,
including the ventrolateral and dorsolateral prefrontal cortices, dorsomedial
prefrontal cortex, insula, cerebellum, and posterior cingulate cortex.
According to the authors:
"The reductions in
gray matter among homicide offenders were evident in a number of brain areas
important for affective processing, social cognition, and strategic behavioral
control."
These latest results agree with some earlier studies and also extend previous
findings.
Most regions identified in the new study appear to have roles
that could be considered pertinent to homicide. For example, the authors
explain that some of these regions are thought to play a role in empathy,
regulating emotions, making moral decisions, assessing others' cognitive
states, and experiencing regrets.
As ever, the study has certain limitations. For instance,
although the analysis took a range of factors into account, there could be
other parameters that the researchers did not measure. As the authors mention,
they did not measure impulsivity — perhaps these neuroanatomical changes are
simply a sign that people who murder are more impulsive than those who carry
out less violent crimes.
Also, this study looked at brain scans at one point in time, so
it cannot determine when these changes arose; were the convicted murdered born
with deficiencies in these areas, or did they develop over time?
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