Brain imaging studies of violent criminals are difficult to interpret because the most persistent among them — those who are responsible for a disproportionate amount of all crime — are not only violent but also overwhelmingly addicted to alcohol and other drugs. That makes it hard to work out which brain changes are associated with addiction (which isn’t usually associated with violence) and which changes are connected with brutal behavior.
A new study aimed to tease out the differences by comparing four groups of volunteers: violent offenders who were addicted to drugs; the rare violent offenders who were not; nonviolent people with alcohol or other drug problems; and those who were neither violent nor addicted.
(More on TIME.com: An Addict’s Battle With Painkiller Addiction Reveals Outdated Rehab Tactics)
These brain scans suggest that drug misuse is linked with reduced brain volume in areas of the cortex that are involved with self-control. Violence, aggression and higher scores on measures of psychopathy, in contrast, were shown to be connected with greater volume in midbrain regions involved with desire, craving, pleasure and motivation.
“Gray matter volumes of the prefrontal cortex were reduced due to substance use disorders, not violence or psychopathy as has been previously suggested,” says Boris Schiffer, lead author of the study and a forensic psychiatrist at the University of Duisburg-Essen in Germany.
In other words, violent brains may be characterized by a more powerful “engine” driving them to act impulsively, while the addicted brain is typified by weaker “brakes” on such drives. In violent offenders who are also addicted to drugs, the combination of a more powerful engine and weaker brakes may leave them with both greater desire to seek pleasure (whether that’s drugs, violence or other activities) and a reduced ability to “just say no.”
(More on TIME.com: The Science of Evil)
“It’s a very interesting result,” says James Blair, chief of affective cognitive neuroscience at the National Institute on Mental Health, who was not associated with the research. However, he notes that earlier research on boys with psychopathic tendencies has shown differences in the brain’s cortex. “We know that there are problems in the orbitofrontal cortex [a region involved in empathy and impulse control] in people with psychopathic tendencies because we see them in child populations that are not exposed to significant drug use,” Blair says.
He adds, “[The research] does suggest that these problems may be exacerbated as children become more exposed to drugs and that makes treatment progressively more difficult.”
In the current study, 75% of the violent offenders met criteria for antisocial personality disorder (ASPD), a condition involving callous disregard for others, ongoing criminal behavior and self-centeredness. But none of the 51 men who were studied met the full criteria for psychopathy.
Psychopathy is the extreme form of ASPD. In psychopaths, there is no empathy, but chronic cruel, violent or manipulative behavior, with complete disregard for the law and the feelings or needs of others. Psychopathy and ASPD may turn out to have different underlying pathology.
(More on TIME.com: Mind Reading: When You Go Hunting for Psychopaths, They Turn Up Everywhere)
Blair notes that the larger midbrain regions found in the violent group in the new study may be more characteristic of hot-headed criminals, people who may be driven to violence by anger and frustration rather than a lack of ability to care for others. Cold, detached psychopaths, in contrast, may simply fail to recognize their victims’ fear and pain.
Previously, researchers have wondered whether psychopathy was also associated with changes in the midbrain, namely in the size of the amygdala, a region that is involved in fear and other emotional processing. (Why? Because studies suggest psychopaths have lower levels of fear: when shown bloody or gory images, psychopaths respond with interest rather than disgust or distress.)
Blair says the studies have generally found reductions or no difference in amygdala size, but Schiffer thinks that the prior research may have been confounded by drug misuse. But because his study did not include full-fledged psychopaths, it cannot shed further light on the question.
However, Schiffer’s study did find that violence — but not drug use — was associated with reduced volume in the left insula, an area involved with disgust, anticipation of negative consequences and understanding the feelings of others. This is in line with previous research suggesting that callous behavior may involve both reduced empathy and less fear of punishment.
(More on TIME.com: How Not to Raise a Bully: The Early Roots of Empathy)
Unfortunately, this type of brain imaging study cannot determine cause and effect, so it can’t show whether these brain differences led to addiction and violence or whether the drug use and violent behavior preceded the changes. In general, research suggests that both genetic influences and environmental factors contribute to addiction and antisocial disorders. Child abuse, neglect and other trauma increase risk for both conditions, and childhood exposure to violence at home or in the community is a particular risk factor for adult violence.
The research was published in the Archives of General Psychiatry.