For decades scientists have been exploring the minds of criminals, trying to understand what makes a person commit a violent, heinous act. The advent of modern brain imaging techniques in the late 20th century proved a boon to the burgeoning field of neurocriminology. Examining the brains of murderers, psychopaths and other violent offenders has unsurprisingly proved to be a controversial scientific pursuit. To some, modern attempts to associate criminal behavior with objective brain imaging skirts discomfortingly close to the controversial 19th century pseudoscience of phrenology.

Phrenology asserted that the shape and contours of a person's skull could be directly associated with a variety of personality traits, from one's intelligence to their propensity for criminal activities. Although the "science" of phrenology was pretty widely discredited by the mid-1800s, its central notion has lingered as a supposed scientific basis for many racist ideas into the 20th century.

Most modern neuroscience looking to find structural or biological signs of criminal and antisocial behaviors in brain imaging data concentrates on more general traits such as aggression or lack of empathy. The neurology of psychopathy is a rich field of neuroscience, with many researchers uncovering fascinating insights into potential brain structures that could underlie certain violent antisocial behaviors.

A new study, published in the journal Brain Imaging and Behavior, is taking this idea of neuroimaging criminal behavior one step further than any prior research. This new research, based on brain imaging data from 808 incarcerated male adults, suggests the brains of individuals that have committed murder are notably different from other criminals, both violent and non-violent.

Specifically imaging the brains of homicide offenders in not especially novel. Pioneering neurocriminologist Adrian Raine conducted some of the earliest neuroimaging studies on murderers in the 1990s, homing in on several brain regions that seemed to be related to homicidal behaviors. However, as corresponding author on this new study Kent Kiehl tells New Atlas, much of this prior neuroimaging work commingled homicidal behavior with other psychiatric conditions such as schizophrenia or psychopathy.

"These early studies relied on murderers who had been found not guilty by reason of insanity (NGRI), and thus include effects of comorbid psychosis and organic brain injury alongside those effects specifically related to homicidal behavior," Kiehl tells us via email. "So yes, our study is novel in that we excluded men with psychosis and brain injury. We also studied nearly 1,000 total offenders in this paper. By far the largest study to date."

The research looked at structural MRI data from subjects categorized into three groups: 203 "homicidal" subjects (including self-reported homicide offenses and explicit attempted murder convicts), 475 violent but non-homicidal subjects (including assault, domestic violence, and other cases consisting of serious bodily harm), and 130 minimally violent subjects (drug possession, prostitution and other crimes resulting in no serious injury to others).

The results strikingly found a number of gray matter differences in the brains of those subjects who committed a homicide, compared to the other two groups. In fact, the researchers suggest there was little difference between the violent but non-homicidal subjects and the minimally violent subjects, implying there are significant gray matter reductions in several brain areas that particularly distinguish homicide offenders from other kinds of criminals.

"The orbital frontal cortex and anterior temporal lobes showed the largest effect sizes; that is, men who committed homicide had less gray matter in these regions than other violent or nonviolent offenders," Kiehl tells us.

These results are arguably rather strange considering the heterogeneous nature of homicide. The study does note it only included serious homicide offenses in the homicide cohort. So accomplices to murder were not included, nor were possible accidental death cases. However, this still strangely mashes up over 200 homicide offenders into a single group. Intent, for example, is not clearly accounted for here, so the study implies little neurological difference between a homicide that resulted from an aggravated assault gone wrong and a more violent, intentional act of murder.

The study does clearly note, "it should not be mistaken for the ability to identify individual homicide offenders using brain data alone, nor should this work be interpreted as predicting future homicidal behavior."

Although, directly questioning Kiehl on this point, he suggests predicting future homicidal behavior from brain data is a reasonable future outcome from this kind of work … and the more data that can be gathered, the more accurate these predictions may ultimately be.

"Yes, this is a first step towards using neuroscience to help predict who will commit homicidal behavior and identifying neuro-risk factors for homicidal behavior," Kiehl tells us. "Our future goals include following up large samples of high risk youth to see if the regions we have identified in this study do predict future homicidal behavior in those samples."

It is fair to be mildly unsettled at the ultimate implications of this research. Kiehl is gesturing to a degree of biological determinism here, suggesting there could be a specific brain "fingerprint" that identifies a person with the capacity to commit murder. Does this mean brain imaging could become major evidence in future murder trials? Or even more disturbing is the question of what we do with the knowledge that there may be an identifiable kind of homicidal brain?

Neurocriminologist Adrian Raine suggests these kinds of criminal brain fingerprints don't necessarily predict future antisocial behaviors but instead imply a certain greater propensity for these kinds of behaviors. When asked in 2013 whether he would be comfortable subjecting his own children to a brain scan that could preemptively assess violent tendencies, he likens the hypothetical process to a kind of public health crime prevention program, where behaviors may be identified and changed at young ages before they develop.

"If there was the opportunity for screening at school or through a GP programme, would I do it? Well, if my kids had problems, as a parent I would want to know about them and I would want to know how I might deal with them," Raine told The Guardian in 2013. "If you brought in such things as emotion regulation and impulse control, which we know are risk factors for behavior, then to me, as a parent, I would sort of want to know what could be done to help with those."

Kiehl is perhaps a little more reticent to hypothesize on the social or cultural outcomes of his line of research. It may be evasive, or merely the classically cold, blinkered vision of a scientist, and Kiehl accepts the potential directions his research could lead in but simply asserts more work is needed before any confident conclusions can be made.

"This study definitely points in the direction of developing a brain 'fingerprint' for homicidal behavior, but our results need to be replicated and we need to conduct longitudinal studies in order to more fully address this critical issue," Kiehl concludes.

The new study was published in the journal Brain Imaging and Behavior.